Precise definitions for every term used across the library, from the physical layer to vehicle architecture.
109 terms
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109 terms
The bit directly after the CRC delimiter in which receivers confirm that a frame arrived intact. The transmitter sends it recessive, and every node that received the frame without error overwrites it with a dominant bit. If the transmitter still reads recessive, it raises an ACK error and sends the frame again, which is why a lone node on a bench bus repeats the same frame endlessly. A recessive ACK delimiter follows the slot.
See alsoCRCError frameEOFDominant bit
The SAE J1939-81 procedure by which every node takes ownership of an 8-bit source address. A node broadcasts an Address Claimed message (PGN 60928) carrying its 64-bit NAME; if two nodes claim the same address, the one with the lower NAME keeps it and the other must claim a different address or fall silent. This lets units from different suppliers share one truck or machine without a central address plan.
The bit-by-bit contest that decides which node may continue when several start transmitting at the same moment. Each transmitter reads the bus while sending its identifier; a node that sends a recessive bit but reads a dominant one has lost and switches to receiving, without damaging the winning frame. The lowest numerical identifier therefore has the highest priority, and no bus time is lost to collisions. Because every bit must reach all nodes and return within one bit time, arbitration is what ties classic CAN bitrate to bus length.
See alsoDominant bitRecessive bitStandard identifierNominal bitrate
Ethernet physical layers built for vehicles that run full-duplex over a single twisted pair instead of the four pairs of office cabling. 100BASE-T1 and 1000BASE-T1 serve point-to-point links, while 10BASE-T1S is a multidrop bus that can take over some branches from CAN. It carries camera, infotainment and backbone traffic between domain or zone controllers, as well as diagnostics through DoIP.
A development partnership of vehicle manufacturers and suppliers that standardises the software architecture of ECUs. Its Classic Platform defines layered basic software beneath the application: communication stacks, network management, diagnostics and memory services. Standard modules such as CanNm for network management and SecOC for message authentication are why ECUs from different suppliers behave consistently on the same bus.
The control unit that monitors and protects a traction battery. It measures cell voltages, currents and temperatures, balances the cells, calculates state of charge and state of health, and opens the main contactors when a limit is exceeded. The BMS shares its values with the vehicle control unit and the charger over CAN or CAN FD.
See alsoREADY modeHVILDC-DC converterECU
The CAN FD control bit that selects whether the data phase runs faster than the arbitration phase. When BRS is recessive, every node switches from the nominal bitrate to the data bitrate at its sample point and switches back at the CRC delimiter; when it is dominant, the whole frame stays at the nominal bitrate. A CAN FD frame without bitrate switching still benefits from the larger payload.
A coding rule that keeps edges on the bus frequent enough for every node to stay synchronised. After five consecutive bits of the same level, the transmitter inserts one bit of the opposite level, which receivers strip out again; six equal bits in a row are therefore a stuff error, the deliberate violation that error flags rely on. Stuffing runs from start of frame to the end of the CRC sequence, so a frame's length on the wire depends on its content. CAN FD adds a stuff-bit counter and fixed stuff bits in the CRC field to guard against bit errors that shift the stuffing.
See alsoError frameCRCSOFCAN FD
The division of each bit into time segments that sets where a node samples the bus and how it stays aligned. A bit consists of a synchronisation segment, a propagation segment and two phase segments, each a whole number of time quanta, with the sample point between phase segments 1 and 2. All nodes on a network need compatible bit timing, not merely the same bitrate; mismatched sample points are a classic cause of intermittent errors.
See alsotqSample pointSJWNominal bitrate
The share of a bus's capacity that frames occupy over a given time window, expressed as a percentage. Stuff bits and interframe space take time too, so it is calculated from the bits actually on the wire rather than from the payload alone. Designers keep sustained bus load well below full capacity so that lower-priority messages still arrive within their deadlines.
The error state in which a CAN node has disconnected itself from the bus because its transmit error counter passed 255. A bus-off node may neither send nor acknowledge frames, and it can only rejoin after observing 128 occurrences of 11 consecutive recessive bits, usually at its software's request. Bus-off keeps a faulty node, for example one set to the wrong bitrate or with a shorted transceiver, from blocking the network for everyone else.
See alsoTEC/RECError passiveError frame
The protocol engine that builds, sends, receives and checks CAN frames on behalf of the host microcontroller. It handles arbitration, bit stuffing, CRC, acknowledgement, error counting and acceptance filtering in hardware and exchanges digital TX and RX signals with the transceiver. Most automotive microcontrollers integrate several controllers, many of them CAN FD capable.
The bus wire whose voltage rises when a dominant bit is sent. In high-speed CAN it rests at about 2.5 V in the recessive state and is driven to about 3.5 V for a dominant bit, while CAN-L moves the opposite way. CAN-H is pin 6 of the OBD connector; on a bus carrying traffic, a multimeter from CAN-H to ground typically reads slightly above 2.5 V.
The complementary bus wire to CAN-H, whose voltage falls when a dominant bit is sent. It rests at about 2.5 V when recessive and is pulled to about 1.5 V for a dominant bit, so the pair carries information as a voltage difference rather than as a level against ground. CAN-L is pin 14 of the OBD connector; on a busy bus, a multimeter to ground typically reads slightly below 2.5 V.
The chip that connects a CAN controller's digital TX and RX lines to the differential bus wires. It drives CAN-H and CAN-L for dominant bits, converts the differential bus voltage back into a logic signal and protects the controller against bus faults, transients and ESD. Modern transceivers add low-power modes, wake-up on bus activity, a dominant time-out and, in some cases, selective wake-up or signal improvement.
The second generation of CAN, standardised in ISO 11898-1:2015. It raises the payload from 8 to 64 bytes and can switch to a faster bitrate for the data phase, while the arbitration phase stays identical to classic CAN. CAN FD controllers also handle classic frames, but a classic-only controller treats a CAN FD frame as an error, so mixed networks need care. CAN FD now carries powertrain, chassis and battery traffic in many new vehicle platforms.
The third generation of CAN, specified by CAN in Automation in CiA 610-1 and included in ISO 11898-1:2024. It carries up to 2,048 bytes per frame and reaches data-phase bitrates of around 20 Mbit/s with suitable transceivers, while keeping CAN's priority-based arbitration. CAN XL is designed to close the gap between CAN FD and 10BASE-T1S Ethernet, and it can carry Ethernet frames as payload.
A higher-layer protocol for industrial automation and machinery, maintained by CAN in Automation as CiA 301. It organises each device around an object dictionary and uses process data objects (PDO) for real-time data, service data objects (SDO) for configuration and network management (NMT) commands to start and stop nodes. CANopen typically uses 11-bit identifiers with a node ID from 1 to 127 embedded in each.
See alsoCANJ1939Standard identifier
The number of pulses the vehicle's speed signal delivers per kilometre travelled, measured by a workshop under defined tyre-pressure and load conditions. It depends on the sensor, the gearing and the effective tyre circumference, known as l. The tachograph's constant k is set equal to w so that the recorded distance matches the distance actually driven.
See alsoConstant kTachographPulse output
The impedance a signal sees as it travels along a uniform cable, set by the cable's geometry and insulation rather than its length. CAN cabling is specified at about 120 Ω, and terminating each end with a resistor of the same value absorbs the signal instead of reflecting it. Sections of very different impedance, long stubs or missing terminations create reflections that show up as ringing on an oscilloscope.
The original CAN protocol, defined as CAN 2.0A with 11-bit identifiers and CAN 2.0B with 29-bit identifiers and now part of ISO 11898-1. A classic frame carries 0 to 8 data bytes at a single bitrate of up to 1 Mbit/s; passenger cars commonly run 500 kbit/s and trucks 250 kbit/s. Classic CAN remains in very wide use, often side by side with CAN FD in the same vehicle.
The average of the CAN-H and CAN-L voltages, about 2.5 V on a healthy high-speed bus. Receivers evaluate only the difference between the wires, so the bus tolerates common-mode shifts caused by ground offsets between ECUs; ISO 11898-2 requires transceivers to work over at least −2 V to +7 V. A common-mode level outside that range, or one that moves with engine load, points to grounding problems.
The setting of a tachograph or taximeter that states how many input pulses the instrument must receive to register one kilometre, expressed in pulses per kilometre. At installation it is set to match the vehicle's characteristic coefficient w, so that the distance and speed the instrument records match reality. New tyres, a different final drive or a changed signal path can alter w, which is why workshops recheck the constant at periodic inspections.
See alsoCharacteristic coefficient wTachographTaximeterPulse output
A multi-master serial bus on which ECUs exchange short, prioritised messages over one shared twisted pair. Developed in the 1980s for in-vehicle networking and standardised as ISO 11898, it addresses messages by identifier rather than by station, so every node receives every frame and decides for itself what to use. Non-destructive arbitration, rigorous error handling and low cost made it the backbone of cars, trucks, agricultural machinery and industrial equipment.
A check value computed by polynomial division over a frame's contents and sent at its end, so that receivers can reject corrupted frames. Classic CAN uses a 15-bit CRC; CAN FD uses 17 bits for payloads of up to 16 bytes and 21 bits above that, together with a stuff-bit count. A receiver that computes a different value reports a CRC error, and the frame is discarded by every node.
See alsoBit stuffingError frameACKCAN FD
The CAN frame type that carries application data from one transmitter to all receivers. It consists of the start of frame, the arbitration field with the identifier, the control field with the DLC, up to 8 data bytes in classic CAN or 64 in CAN FD, the CRC field, the ACK field and the end of frame. Remote, error and overload frames are the protocol's other frame types.
A 16-bit number that addresses one data record in an ECU over UDS, read with service 0x22 and written with service 0x2E. Some ranges are standardised, such as F190 for the vehicle identification number, while most DIDs are defined by the vehicle manufacturer. DIDs are the UDS counterpart of OBD PIDs.
The 4-bit field in the control field that states how many data bytes a frame carries. In classic CAN, values 0 to 8 map directly to the byte count, and 9 to 15 also mean 8. In CAN FD, codes 9 to 15 stand for 12, 16, 20, 24, 32, 48 and 64 bytes, so payloads above 8 bytes are padded to the next valid size.
See alsoData frameCAN FDClassic CAN
The part of a CAN FD frame in which the faster data bitrate may be used, from the sample point of the BRS bit to the CRC delimiter. Only one node transmits during this phase, so its bit time no longer has to allow for a signal round trip across the network, and that is what makes the higher speed possible. Data-phase bitrates of 2 Mbit/s and 5 Mbit/s are common in vehicles.
See alsoBRSNominal bitrateTDCCAN FD
In electric and hybrid vehicles, the converter that steps the high-voltage traction battery down to the low-voltage system and takes over the alternator's role. While the vehicle is in READY mode it charges the 12 V battery and supplies the low-voltage loads. Once the high-voltage system shuts down, everything falls back on the 12 V battery alone.
See alsoREADY modeBMSSleep current
A code that an ECU stores when it registers a fault, together with status bits showing whether the fault is active, pending or confirmed. OBD codes begin with a letter for the system (P powertrain, C chassis, B body, U network) followed by four characters; U0100, for example, reports lost communication with the engine control module. Network faults such as bus-off or missing messages typically set U codes in several ECUs at once.
ISO 13400, the transport of UDS diagnostic messages over Ethernet and TCP/IP. It lets workshop equipment diagnose and reprogram ECUs far faster than over CAN and is reached through additional pins of the OBD connector or a dedicated diagnostic port. DoIP traffic normally ends at the vehicle gateway, which relays each request to the target ECU.
Transmitting a bit as the voltage difference between two wires rather than as a level against ground. Interference couples almost equally into both wires of a twisted pair and cancels out when the receiver subtracts them, which is why CAN keeps working in an electrically noisy vehicle. In high-speed CAN, a difference above 0.9 V reads as dominant and one below 0.5 V as recessive.
A high-performance ECU that consolidates the functions of one vehicle domain, such as powertrain, chassis, body, driver assistance or infotainment, that used to run on many smaller ECUs. Domain controllers usually connect to each other over an Ethernet backbone and to their own ECUs and sensors over CAN, CAN FD or LIN. Many manufacturers are now moving beyond domains to zonal architectures.
The bus state that always wins when nodes transmit different values at the same moment; it represents logical 0. A transmitter creates it by actively driving CAN-H up and CAN-L down, giving a differential voltage of about 2 V. Because one dominant bit overrides any number of recessive ones, arbitration, acknowledgement and error signalling are all built on it.
See alsoRecessive bitArbitrationACKCAN-H
The ability of electronic equipment to work in its electromagnetic environment without disturbing other equipment. For vehicle electronics it covers emissions, assessed for example with CISPR 25, and immunity to radiated fields and conducted transients, assessed with the ISO 11452 and ISO 7637 series. Good EMC starts at installation: short leads, intact twisted pairs, clean grounds and correct termination.
Any control unit in a vehicle that runs software to measure, decide and actuate: engine, brakes, airbags, doors, instruments and many more. A modern car contains anywhere from several dozen to more than a hundred ECUs, which exchange information over CAN, CAN FD, LIN, FlexRay and Ethernet. Every ECU on a CAN network is a node with its own controller and transceiver.
See alsoCAN controllerCAN transceiverGatewayDomain controller
The sudden current that flows when a charged body, often a person, touches or approaches a conductor. Bus pins and connectors exposed to handling, such as the OBD port, need protection components that divert these pulses before they reach the transceiver. ESD tests for road-vehicle electronics are defined in ISO 10605.
See alsoEMCCAN transceiverSAE J1962
The sequence of seven recessive bits that closes every CAN data and remote frame. Bit stuffing does not apply here, and this fixed form lets nodes catch late errors: a dominant bit inside the EOF is a form error. The interframe space follows before the next frame may start.
See alsoSOFIFSError frame
The transmit error counter (TEC) and receive error counter (REC) that every CAN controller keeps to track its own reliability. An error while transmitting usually adds 8 to the TEC, an error while receiving adds 1 to the REC, and each successful frame lowers the relevant counter by 1. The counters decide whether a node is error active, error passive or bus-off, so a node that keeps causing errors gradually withdraws from the bus.
See alsoError passiveBus-offError frame
The frame a CAN node sends as soon as it notices a bit, stuff, CRC, form or acknowledgement error. An error-active node transmits six dominant bits, deliberately breaking the stuffing rule so that every other node notices too, followed by an eight-bit recessive delimiter, after which the original transmitter retries. Occasional error frames are normal in a vehicle; a steady stream of them points to wiring, termination or bitrate problems.
The intermediate error state a CAN node enters when its transmit or receive error counter exceeds 127. Unlike an error-active node, it flags errors with six recessive bits that cannot disrupt other traffic, and it must wait an extra suspend-transmission time between its own frames. When both counters drop below 128 again it returns to error active; if the transmit counter passes 255 it goes bus-off.
See alsoTEC/RECBus-offESIError frame
A bit in the CAN FD control field that tells receivers the transmitter's error state: dominant for error active, recessive for error passive. It makes a struggling node visible to the network before it reaches bus-off. Because ESI follows the BRS bit, it is already sent at the data bitrate when bitrate switching is active.
See alsoError passiveBRSFDFCAN FD
The 29-bit identifier introduced with CAN 2.0B, made of an 11-bit base identifier and an 18-bit extension, with more than 536 million possible values. It is signalled by a recessive IDE bit and is the norm in SAE J1939, ISOBUS and many commercial-vehicle networks. When a standard and an extended frame share the same base identifier, the standard frame wins arbitration.
A 5-bit code in J1939 diagnostic messages that describes how a parameter has failed, for example data above the normal range, voltage below normal or an abnormal update rate. Combined with an SPN, it pinpoints both the affected parameter and the nature of the fault. FMI values 0 to 31 are defined in SAE J1939-73.
The control bit that distinguishes CAN FD frames from classic frames; early specifications called it EDL. In a classic frame this position holds a dominant reserved bit, while in a CAN FD frame FDF is recessive and is followed by a further reserved bit, then BRS and ESI. A classic-only controller cannot follow the rest of a CAN FD frame and answers it with error frames, which is why CAN FD traffic cannot share a bus with active classic-only nodes.
An open interface agreed by European truck and bus manufacturers through which telematics equipment receives a defined set of J1939 vehicle data. The vehicle provides it through a gateway that passes only the published messages, such as fuel, distance, engine hours and driver working states, and accepts no commands in return. A Bus-FMS variant covers buses and coaches.
See alsoJ1939GatewayTachograph
A deterministic vehicle bus standardised as ISO 17458, running at up to 10 Mbit/s on each of two channels. Communication follows a fixed cycle with a static segment of time-triggered slots and a dynamic segment for event-driven messages, so the timing of every message is known in advance. FlexRay served chassis and by-wire applications that needed that determinism; newer platforms increasingly use CAN FD and Ethernet instead.
See alsoCAN FDAutomotive EthernetLIN
An adapter that plugs into a fuse slot and provides an additional, separately fused output alongside the original circuit. It is a tidy way to take terminal 15 or terminal 30 power for an aftermarket device without cutting into the harness, provided the original circuit's rating and switching behaviour suit the new load. Fitting the tap the right way round keeps both circuits protected by their own fuse.
See alsoKL15KL30Sleep current
Separation of two circuits so that no direct current path exists between them, while data or power still crosses through transformers, capacitors or optocouplers. Isolated CAN transceivers protect equipment and break ground loops when devices at different ground potentials are connected, a vehicle and a mains-powered bench computer for instance. Isolation is standard practice in high-voltage battery systems and test equipment.
An ECU that connects several vehicle networks and forwards messages between them, converting bitrates, protocols and frame formats as needed. The central gateway separates domains such as powertrain, body and infotainment so that faults or traffic on one bus do not spill into another. It is often the only path from the OBD connector to the internal networks, and its routing configuration determines which messages appear on which bus.
See alsoSGWOBD-IIDomain controllerDoIP
An unwanted current path that forms when two points of a system are grounded at different places with different potentials. The resulting current flows through shields, signal grounds or bus references and appears as noise, common-mode shifts or communication errors. In vehicles, ground loops typically arise when test equipment or aftermarket devices are grounded somewhere other than the ECUs they communicate with.
A sensor that switches or produces a voltage when a magnetic field passes it, used to count the teeth of a rotating tone wheel. Wheel speed, gearbox and tachograph motion sensors often rely on the Hall effect because it works down to standstill and delivers a clean digital output. Many such sensors have an open-collector output that needs a pull-up resistor.
See alsoVSSOpen collectorPulse output
A low-voltage circuit looped through every high-voltage connector and cover of an electric or hybrid vehicle. If a connector is unplugged or a cover opened, the loop breaks and the battery management system opens the main contactors, removing high voltage from the system. HVIL faults are a common reason why a vehicle refuses to enter READY mode.
See alsoBMSREADY modeDC-DC converter
The bit that tells receivers whether a frame uses an 11-bit or a 29-bit identifier. A dominant IDE marks a standard frame; a recessive IDE marks an extended frame, whose remaining 18 identifier bits follow. In extended frames, the position of the standard RTR bit is taken by the substitute remote request (SRR) bit.
The code from IEC 60529 that rates an enclosure's protection against solid objects and water, written as IP followed by two digits. The first digit (0–6) covers dust and contact, the second (0–9) water; IP67, for instance, means dust-tight and protected against temporary immersion. ISO 20653 adapts the system to road vehicles and adds 9K for high-pressure, high-temperature washdown.
The minimum gap between CAN frames: three recessive intermission bits after the end of frame, followed by bus idle. No node may start a data frame during intermission, which gives controllers time to process the frame they have just received. An error-passive node that has just transmitted must also wait eight bits of suspend transmission.
See alsoEOFSOFError passive
The international standard for the CAN data link layer and physical signalling: frame formats, arbitration, bit stuffing, error handling and bit timing. The 2015 edition added CAN FD and the 2024 edition added CAN XL. It is the reference every CAN controller implements.
The standard for the high-speed CAN physical medium attachment, in other words the transceiver and its bus levels. It defines recessive and dominant voltages, common-mode range, timing and low-power modes, and since its 2016 edition it also covers selective wake-up, formerly ISO 11898-5 and -6. High-speed CAN built on it runs at up to 1 Mbit/s for classic frames and at CAN FD data bitrates of several Mbit/s.
See alsoCAN transceiverISO 11898-1ISO 11898-3Partial networking
The standard for low-speed, fault-tolerant CAN, used at up to 125 kbit/s in body and comfort networks. Each node carries its own termination, and the bus keeps communicating, with reduced noise immunity, even if one wire is broken or shorted to ground or battery. Its bus levels differ from high-speed CAN, so the two cannot be connected directly.
The transport protocol that carries messages longer than one CAN frame, such as UDS requests and responses. It splits a message into a first frame and numbered consecutive frames while the receiver paces the transfer with flow-control frames; short messages fit into a single frame. With CAN FD, a single frame can carry up to 62 bytes of payload.
See alsoUDSISO 15765-4DoIP
The standard that defines how emissions-related OBD diagnostics run on CAN. It fixes the bitrate at 250 or 500 kbit/s, the identifiers (functional requests on 7DF with responses from 7E8 to 7EF in the 11-bit scheme, or 18DB33F1 in the 29-bit scheme) and the timing a diagnostic tool must follow. Since model year 2008, every new car sold in the United States has had to use it for OBD.
The CAN-based standard ISO 11783 that lets tractors and implements from different manufacturers work together. Built on SAE J1939 at 250 kbit/s, it adds services such as a virtual terminal that brings implement controls into the cab, a task controller for job documentation and a tractor ECU that shares tractor data with the implement. Interoperability between brands is checked through the industry's shared conformance testing.
A single-wire, bidirectional diagnostic line defined in ISO 9141 and used by the KWP2000 protocol (ISO 14230), typically at 10.4 kbit/s. It sits on pin 7 of the OBD connector and remains common on older vehicles and some motorcycles. On newer vehicles, CAN-based diagnostics have largely replaced it.
The voltage surge that occurs when the battery is disconnected from a charging alternator, for example by a loose terminal, and the alternator's stored energy drives the supply far above normal for up to several hundred milliseconds. ISO 16750-2 describes it as test pulse 5a (unsuppressed) and 5b (with central clamping); in 12 V systems the unsuppressed pulse can reach 87 V. Every device on the vehicle supply must withstand it, typically with transient-voltage-suppression diodes and robust input stages.
A low-cost, single-wire bus standardised as ISO 17987 and running at up to 20 kbit/s. One commander node schedules all communication and polls responder nodes such as switches, window lifters, mirrors, seat motors or sensors. LIN sub-networks typically hang off a body ECU that links them to CAN.
EU Directive 2014/32/EU, which sets the essential requirements for measuring instruments placed on the market, with taximeters covered in annex MI-007. A taximeter must pass conformity assessment before sale and carries the supplementary metrology marking, a capital M with the year, to show it. National rules then govern installation, sealing and periodic verification in the vehicle.
See alsoTaximeterConstant kPulse output
The coordination through which ECUs on a bus agree when the network may sleep and when it must stay awake. In AUTOSAR network management, each node sends periodic NM messages while it needs the bus; when no node has sent one for a defined time, all nodes enter bus-sleep together. A single device that keeps transmitting or wakes the bus repeatedly stops the whole network from sleeping and raises the vehicle's sleep current.
The bitrate of the arbitration phase of a CAN frame and, in classic CAN, of the entire frame. It must be identical on every node, typically 500 kbit/s in passenger cars and 250 or 500 kbit/s in commercial vehicles. Higher bitrates shorten the maximum bus length: roughly 40 m at 1 Mbit/s, 100 m at 500 kbit/s and 250 m at 250 kbit/s.
The standardised self-diagnosis that emissions regulations require in road vehicles, together with its 16-pin connector and a common set of diagnostic services. It became mandatory in the United States for model-year 1996 cars and in Europe, as EOBD, for petrol cars from 2001 and diesel cars from 2004. Any generic diagnostic tool can read emissions-related fault codes and live data, while access to other systems depends on the manufacturer.
See alsoSAE J1962ISO 15765-4PIDDTCSGW
An output stage whose transistor can only pull the line to ground and leaves it floating when off. An external pull-up resistor to the desired supply sets the high level, so the same output can drive 5 V, 12 V or 24 V inputs. Open-collector outputs, called open-drain when built with MOSFETs, are common for pulse signals and also let several outputs share one line.
An instrument that displays voltage over time and the most direct way to judge the physical health of a CAN bus. Measuring CAN-H and CAN-L against ground, or the difference between them, reveals levels, edge shapes, ringing from reflections and noise that a multimeter averages away. Clean, square bits with matching dominant levels from every node indicate a healthy physical layer.
See alsoCharacteristic impedanceCAN-HCAN-LTermination resistor
The 18-bit number in a J1939 identifier that names a group of related parameters, such as engine temperatures or vehicle distance. It is formed from the extended data page, data page, PDU format and PDU specific fields; when the PDU format is below 240, the PDU specific field carries a destination address instead. Each PGN definition fixes the message's length, transmission rate, default priority and the SPNs it contains.
The one-byte code that selects a data item in OBD service 01 (current data) or 02 (freeze-frame data), defined in SAE J1979. For example, 05 returns engine coolant temperature and 0C engine speed, while PID 00 returns a bitmap of the PIDs the ECU supports. Every emissions-relevant ECU answers the PIDs it supports in the same format, whatever the make.
See alsoOBD-IIISO 15765-4DIDDTC
A power-saving technique in which only the ECUs a function needs are woken while the rest of the network stays asleep. Transceivers with selective wake-up, defined in ISO 11898-2, remain in low-power mode, decode incoming frames and wake their ECU only for a configured wake-up frame. It cuts energy use in parked vehicles and during charging.
A resistor that connects a signal line to a supply voltage so that the line rests at a defined high level whenever no output pulls it low. Open-collector outputs depend on it, and its value balances edge speed, current draw and noise immunity, typically somewhere between about one and a few tens of kilohms. Without a pull-up, an open-collector line floats and picks up interference.
See alsoOpen collectorPulse output
A square-wave signal whose pulse count is proportional to the distance travelled, so its frequency is proportional to speed. Tachographs, taximeters and many fleet and body-builder devices accept such signals, specified by pulses per kilometre, voltage level and output type. Matching the receiving instrument's constant k to the pulse rate is what makes its distance reading correct.
The operating state of an electric or hybrid vehicle in which the high-voltage system is live and the vehicle will move as soon as the accelerator is pressed, usually indicated by a READY lamp. Because the engine may be off or absent altogether, a silent vehicle in READY mode is fully driveable. Its ECUs and buses are active in this state, unlike the ignition-on, not-ready state used for accessories or diagnostics.
See alsoKL15DC-DC converterBMSHVIL
The passive bus state, representing logical 1, that appears whenever no node drives the bus. In high-speed CAN both wires then sit near 2.5 V, set by the transceivers' bias, while the termination pulls the difference between them close to zero. Any node sending a dominant bit overrides it, so the bus only shows a recessive bit when every node sends one.
The bit after the identifier that distinguishes a data frame (dominant) from a remote frame (recessive), with which one node asks another to send the data for that identifier. Remote frames are rarely used in vehicles. CAN FD has no remote frames; in this position it carries the always-dominant remote request substitution (RRS) bit.
See alsoData frameIDEStandard identifier
Circuitry that keeps a device from being damaged when its supply is connected the wrong way round, as can happen during installation or a jump start. Common solutions are a series diode or, with lower losses, a MOSFET-based ideal-diode circuit. Vehicle electronics standards such as ISO 16750-2 include a reverse-voltage test.
The family of SAE standards for CAN communication in trucks, buses, and agricultural and construction machinery. It uses 29-bit identifiers that encode priority, a parameter group number and the source address, and runs at 250 kbit/s or, under J1939-14, at 500 kbit/s. Because its messages and parameters are standardised, the same parameter means the same thing across manufacturers.
See alsoPGNSPNAddress claimISOBUSFMS
The standard that defines the 16-pin OBD diagnostic connector and its location within easy reach of the driver's seat. Key pins are 4 (chassis ground), 5 (signal ground), 6 (CAN-H), 14 (CAN-L), 16 (battery positive) and 7 (K-Line), and manufacturers may use the remaining pins for their own purposes. Type A suits 12 V vehicles, while type B, with an interrupted central guide, is used on 24 V vehicles.
The standard for single-wire CAN, which uses one signal wire against ground at 33.3 kbit/s, with an 83.3 kbit/s high-speed mode for diagnostics and reprogramming. It saves wiring in body and comfort networks and was used in particular by some North American manufacturers. Single-wire CAN has its own transceivers and cannot be wired directly to a high-speed CAN bus.
See alsoISO 11898-3CANCAN transceiver
The moment within each bit at which a CAN node reads the bus level, expressed as a percentage of the bit time. A late sample point gives signals time to propagate and settle, and automotive networks typically place it at about 80 to 87.5 % in the arbitration phase. All nodes on a network should use closely matching sample points; otherwise long topologies and higher bitrates produce intermittent errors.
See alsoBit timingtqSJWTDC
A gateway that limits what external tools connected to the OBD port are allowed to do. Reading fault codes and data usually stays open, while clearing codes, actuator tests, coding and programming require an authenticated, authorised tool. Several manufacturers have fitted secure gateways since around 2018, and cybersecurity regulation has made such access controls widespread.
The AUTOSAR module that authenticates individual messages on vehicle buses. The sender appends a truncated message authentication code and a freshness value, and the receiver checks both, so injected or replayed frames are rejected. It protects safety-relevant data against manipulation without encrypting the bus.
A transceiver feature that actively suppresses the ringing that reflections cause after recessive edges, particularly in star-shaped or stub-heavy networks. By briefly driving the bus towards the recessive level with a defined impedance, SIC transceivers allow CAN FD data bitrates of 5 Mbit/s and beyond in topologies that would otherwise top out around 2 Mbit/s. The concept, specified in CiA 601-4, also underpins the CAN XL transceivers.
See alsoCAN transceiverCAN FDCAN XLStub
The current a vehicle draws from its 12 V battery once it is parked, locked and its networks have gone to sleep, also called quiescent current. Manufacturers budget it tightly, ECU by ECU, so that the vehicle still starts after weeks of standing. A device that keeps a bus awake or draws too much from terminal 30 can multiply the sleep current and flatten the battery; measuring it takes patience, because the networks settle only after a timeout.
See alsoNMKL30Wake-upPartial networking
The second-generation digital tachograph defined under Regulation (EU) No 165/2014 and its implementing Regulation (EU) 2016/799, required in newly registered vehicles from June 2019, with version 2 required from August 2023. It adds a short-range remote communication interface (DSRC) for roadside checks, an interface for intelligent transport systems and stronger security between the vehicle unit and its motion sensor. Version 2 also records border crossings and loading and unloading operations.
The 11-bit identifier of CAN 2.0A, with 2,048 possible values, signalled by a dominant IDE bit. It names the content of a frame and, through arbitration, sets its priority: the lower the value, the higher the priority. Most passenger-car networks use standard identifiers.
The single dominant bit that begins every CAN data and remote frame. Its falling edge out of the recessive bus-idle state hard-synchronises all receivers, which then stay aligned by resynchronising on later edges. A node may send SOF only while the bus is idle.
See alsoEOFBit timingArbitration
The branch line between the main bus and an individual node. On a high-speed CAN bus, stubs are unterminated and reflect signals, so they must be kept short, the shorter the higher the bitrate, and a device should join close to the main line rather than through a long extension. Several long stubs together are a frequent cause of errors at CAN FD data bitrates.
The J1939 number that identifies a single parameter, such as engine coolant temperature or fuel level, along with its position, length, resolution, offset and range within its parameter group. The same SPN appears in diagnostic messages, together with an FMI, to name the parameter a fault relates to. SPNs are 19-bit values defined in SAE J1939-71 and related documents.
The maximum number of time quanta by which a CAN controller may lengthen phase segment 1 or shorten phase segment 2 to resynchronise on a bus edge. It compensates for oscillator tolerances between nodes, and a larger SJW tolerates more clock deviation. SJW may not exceed the shorter of the two phase segments.
See alsoBit timingtqSample point
The recording instrument that EU and AETR rules require in goods vehicles above 3.5 t and in passenger vehicles with more than nine seats including the driver, logging driving, working and rest times, distance and speed. Analogue chart recorders gave way to digital tachographs in 2006 and smart tachographs in 2019. Under the EU Mobility Package, light commercial vehicles above 2.5 t used for international transport or cabotage also need a smart tachograph from 1 July 2026.
See alsoSmart tachographConstant kCharacteristic coefficient wPulse output
The measuring instrument that calculates a taxi fare from distance travelled and time under the applicable tariff. It receives a distance signal from the vehicle as pulses and converts it using its constant k, so the fare depends directly on the accuracy of that signal. In the EU, taximeters fall under annex MI-007 of the Measuring Instruments Directive, while national rules govern installation, sealing and periodic verification.
See alsoMIDConstant kPulse outputVSS
The DIN 72552 designation for the positive supply that is live only while the ignition is on. Equipment that should run only while the vehicle is in use is fed from terminal 15, so it cannot drain the battery when parked. In keyless and many electric vehicles, terminal 15 is switched electronically and its state is also shared over the vehicle network; ignition on and READY are not always the same state.
See alsoKL30KL31READY modeWake-up
The DIN 72552 designation for the permanent battery positive that stays live with the ignition off. Loads that must keep working while the vehicle is parked, such as the alarm, memory functions and network wake-up, are fed from terminal 30, so anything connected to it adds to the vehicle's sleep current. Aftermarket devices on terminal 30 should be fused close to the tap and draw minimal current while the vehicle sleeps.
See alsoKL15KL31Sleep currentFuse tap
The DIN 72552 designation for the return path to battery negative or chassis ground. Grounding every device at a solid point close to the ECUs it communicates with keeps bus common-mode levels and sensor references stable. Corroded or shared ground points are a common and hard-to-find source of intermittent faults.
The resistor of about 120 Ω placed between CAN-H and CAN-L at each of the two physical ends of a high-speed bus. It matches the cable's characteristic impedance to suppress reflections and defines the recessive state by discharging the bus. With the supply off, a meter across CAN-H and CAN-L should read about 60 Ω; 120 Ω points to a missing terminator and about 40 Ω to one too many. Split termination divides each resistor into two 60 Ω halves with a capacitor to ground at the centre for better EMC.
The basic unit of CAN bit timing, derived from the controller clock through a prescaler. Every bit consists of a whole number of time quanta, typically 8 to 25 in classic CAN, distributed across the synchronisation, propagation and phase segments. The clock frequency and the prescaler together determine which bitrates and sample points can be set exactly.
See alsoBit timingSample pointSJW
A CAN FD mechanism that lets a transmitter check its own bits in the data phase even when the loop delay through its transceiver exceeds one bit time. Instead of the normal sample point, it checks each bit at a secondary sample point shifted by the measured delay. Without TDC, a transmitter would see bit errors in its own frames at higher data bitrates.
Two insulated wires twisted around each other, the standard cable for CAN, CAN FD and automotive Ethernet. Twisting exposes both wires equally to external fields, so interference couples into them equally and cancels in the differential receiver, while the pair's own emissions cancel as well. Where the pair is untwisted at splices or connectors, that length should be kept as short as possible.
The UNECE regulation that requires vehicle manufacturers to operate a cybersecurity management system and to protect each vehicle type against defined threats, from network intrusion to manipulated diagnostic access. In the EU it has applied to all newly registered vehicles since July 2024. It is one reason why secure gateways and message authentication have become common.
The ISO 14229 diagnostic protocol that modern ECUs use for reading and clearing fault codes, reading and writing data, running routines, security access and reprogramming. A request starts with a one-byte service ID, such as 0x10 for diagnostic session control, 0x22 for read data by identifier or 0x19 for read DTC information; a positive response returns the service ID plus 0x40, a negative one starts with 0x7F. UDS runs over CAN through ISO-TP and over Ethernet through DoIP.
A conventional electrical signal, usually a pulse train, that reports vehicle speed and distance to other equipment. It is typically produced by a gearbox or output-shaft sensor, derived from the wheel speed sensors or taken from the tachograph's pulse output. In many modern vehicles the hard-wired signal no longer exists and speed is shared only as a message on the vehicle network.
The transition of an ECU or network from low-power sleep to normal operation. A CAN transceiver in standby wakes on a defined bus wake-up pattern, on a configured wake-up frame when selective wake-up is used, or on a local input such as a door switch or terminal 15. Every unnecessary wake-up costs battery charge, which is why aftermarket devices should never transmit on a sleeping bus.
An electrical and electronic architecture that groups ECUs and wiring by location in the vehicle rather than by function. Zone controllers gather the sensors, actuators and CAN or LIN networks in their area and connect them over an Ethernet backbone to one or a few central computers. The approach shortens the wiring harness and makes new functions easier to add, while CAN FD and CAN XL remain the local networks inside each zone.
Santim SC-1 supports every classic CAN and CAN FD vehicle on the market. When a new vehicle launches, it is compatible instantly. No waiting, no requests. A next-generation CAN device.
