J1708
Encyclopedia
SAE J1708 is a standard used for serial communications
between ECU
s on a heavy duty vehicle and also between a computer and the vehicle. With respect to Open System Interconnection model (OSI), J1708 defines the physical layer. Common higher layer protocols that operate on top of J1708 are SAE J1587
and SAE J1922
.
The standard defines a 2-wire 18 gauge wire cable that can run up to 130 feet (40 m) and operates at 9600 bit/s. A message is composed of up to 21 characters, unless the engine is stopped and the vehicle is not moving in which case transmitters are allowed to exceed the 21 byte max message length. Messages start with a Message ID (MID) character and finish with a checksum
at the end. Characters are transmitted in the common 8N1 format.
The hardware utilized are RS-485 transceivers wired for open collector
operation through the use of a pullup and pulldown of the separate data lines. Transmission is accomplished by controlling the driver enable pin of the transceiver. This method allows multiple devices to share the bus without the need for a single master node. Collisions are avoided by monitoring the bus while transmitting the MID to ensure that another node has not simultaneously transmitted a MID with a higher priority.
SAE J1708, although still widely used, is replaced by SAE J1939
which is a CAN protocol.
The protocol is maintained by SAE International
.
Serial communications
In telecommunication and computer science, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are sent as a whole, on a link with several parallel channels...
between ECU
Electronic control unit
In automotive electronics, electronic control unit is a generic term for any embedded system that controls one or more of the electrical systems or subsystems in a motor vehicle....
s on a heavy duty vehicle and also between a computer and the vehicle. With respect to Open System Interconnection model (OSI), J1708 defines the physical layer. Common higher layer protocols that operate on top of J1708 are SAE J1587
J1587
J1587 is an automotive diagnostic protocol standard developed by the Society of Automotive Engineers for heavy-duty and most medium-duty vehicles built after 1985. The J1587 protocol uses different diagnostic connectors. Up to 1995, individual OEMs used their own connectors. From 1996 to 2001, the...
and SAE J1922
J1922
SAE protocol J1922 is a standard for "Powertrain Control Interface for Electronic Controls Used in Medium- and Heavy-Duty Diesel On-Highway Vehicle Applications"...
.
The standard defines a 2-wire 18 gauge wire cable that can run up to 130 feet (40 m) and operates at 9600 bit/s. A message is composed of up to 21 characters, unless the engine is stopped and the vehicle is not moving in which case transmitters are allowed to exceed the 21 byte max message length. Messages start with a Message ID (MID) character and finish with a checksum
Checksum
A checksum or hash sum is a fixed-size datum computed from an arbitrary block of digital data for the purpose of detecting accidental errors that may have been introduced during its transmission or storage. The integrity of the data can be checked at any later time by recomputing the checksum and...
at the end. Characters are transmitted in the common 8N1 format.
The hardware utilized are RS-485 transceivers wired for open collector
Open collector
An open collector is a common type of output found on many integrated circuits . Instead of outputting a signal of a specific voltage or current, the output signal is applied to the base of an internal NPN transistor whose collector is externalized on a pin of the IC. The emitter of the...
operation through the use of a pullup and pulldown of the separate data lines. Transmission is accomplished by controlling the driver enable pin of the transceiver. This method allows multiple devices to share the bus without the need for a single master node. Collisions are avoided by monitoring the bus while transmitting the MID to ensure that another node has not simultaneously transmitted a MID with a higher priority.
SAE J1708, although still widely used, is replaced by SAE J1939
J1939
Society of Automotive Engineers SAE J1939 is the vehicle bus standard used for communication and diagnostics among vehicle components, originally by the car and heavy duty truck industry in the United States....
which is a CAN protocol.
The protocol is maintained by SAE International
SAE International
SAE International is an organization for engineering professionals in the aerospace, automotive, and commercial vehicle industries. The Society is a standards development organization for the engineering of powered vehicles of all kinds, including cars, trucks, boats, aircraft, and others.SAE...
.