RP 1201 - Joint TMC/Sae Recommended Practice For Serial Data Communications Between Microcomputer Systems In Heavy-Duty Vehicle Application: Protocol
Bit Time - A bit time shall be 104.17 μs ±0.5% (±500 ns). This is equivalent to a baud rate of 9600 bits per second.
Character Format - A character shall consist of 10 bit times The first bit shall always be a low logic level and is called the start bit. The last (tenth) bit shall always be a high logic level and is called the stop bit. This convention is consistent with standard UART operation. The remaining eight character bits are data bits that are transmitted least significant bit (LSB) first.
Message Content - A message appearing on the communication bus shall consist of a message identification character, data characters, and a checksum. A message shall always be proceeded by an idle state of duration equal to or greater than the appropriate access time. The length of time between characters within a message shall not exceed two bit times.
Message Identification Character (MID) - The first character of every message shall be a MID. The permitted range of MIDs shall include the numbers 0 to 255. The MIDs 0 to 68 shall be assigned to transmitter categories as identified in MESSAGE IDENTIFICATION CHARACTER ALLOCATION. These assignments have been made to accommodate existing systems or systems that may presently be underdevelopment and to avoid conflicts which otherwise might arise if indiscriminate use of MIDs were permitted.
MIDs 69 to 86 have been set aside for use by SAE J1922.
MIDs 87 to 110 shall be allocated as reserved MIDs for transmitter categories beyond those that are identified in MESSAGE IDENTIFICATION CHARACTER ALLOCATION. These MIDs shall be individually assigned by the SAE Electronics Interface Subcommittee of the SAE Truck and Bus Electrical Committee on petition by a manufacturer at the time a new transmitter category is identified or when additional MIDs are required within a previously identified category. The content and format of these messages using the assigned MIDs (0 to 110) is the responsibility of the transmitter. The content of the data format within these messages is not defined in this document, but should be identified in an appropriate applications document.
| MID RANGE | TRANSMITTER CATEGORY |
|---|---|
| 00-07 | Engine |
| 08-09 | Brakes, Tractor |
| 10-11 | Brakes, Trailer |
| 12-13 | Tires, Tractor |
| 14-15 | Tires, Trailer |
| 16-17 | Suspension, Tractor |
| 18-19 | Suspension, Trailer |
| 20-27 | Transmission |
| 28-29 | Electrical Charging System |
| 30-32 | Electrical |
| 33-35 | Cargo Refrigeration/Heating |
| 36-40 | Instrument Cluster |
| 41-45 | Driver Information Center |
| 46-47 | Cab Climate Control |
| 48-55 | Diagnostic Systems |
| 56-61 | Trip Recorder |
| 62-63 | Turbocharger |
| 64-68 | Off-Board Diagnostics |
| 69-86 | Set Aside For SAE J1922 |
| 87-110 | Reserved - To Be Assigned By Electronic Interface Subcommittee |
| 111 | Reserved - Factory Electronic Module Tester (Off Vehicle) |
| 112-127 | Unassigned - Available For Use |
| 128-255 | To Be Assigned By The Data Format Subcommittee |
MID 111 shall be used exclusively for the factory test of electronic modules. Since it is possible that during the factory test the normal control software is bypassed giving the tester direct control of module I/O, the following precautions should be observed:
- Entry into the factory test should be granted by the module control software only after ensuring that it is safe to do so.
- MID 111 should not be transmitted by any on-board module.
MIDs 112 to 127 are not assigned to any category and are not reserved for future assignment. These MIDs are available to any manufacturer or user for any message identification purpose outside the scope of this Recommended Practice.
MIDs in the 0 to 68 and 87 to 127 ranges shall be defined in the manufacturer's applications document. It shall be the responsibility of the systems integrator or user to ensure that a particular MID is not used by more than one device on the same vehicle.
MIDs in the range of 128 to 255 shall be reserved for applications using formatted data as set forth in a document issued by the SAE Truck and Bus Electrical Committee Data Format Subcommittee. These MIDs shall only be used when the data format set forth within that document is strictly followed. See SAE/TMC Joint Recommended Practice J1587.
Data Characters - Data characters shall be characters that convey the intelligence of the message and shall conform to the character format as defined in MESSAGE IDENTIFICATION CHARACTER ALLOCATION. The eight-bit data characters may be given any value from 0 to 255. The data characters shall be defined in an appropriate applications document at the option of the supplier. The application document shall define parameters, parameter order, scaling, and error detection and correction coding, if applicable.
Checksum - The last character of each message shall be the two's complement of the sum of the MID and the data characters. Simple error detection may be implemented by adding the checksum to the sum of all previous message characters (including the MID). The eight-bit sum will be zero, neglecting the CARRY, for a correctly received message.
Message Length - Total message length, including the MID and checksum, shall not exceed 21 characters. Exceptions to this length limitation may be made when the engine is not running and the vehicle not moving. Messages longer than 21 characters may also be broken up into several separate messages of 21 or fewer characters and may then be transmitted while the engine is running and/or the vehicle is moving by conforming to the 21 character message length limitation of SAE J1708.