SAE j1708/j1587 protocol details

One , brief introduction

SAE J1708 and SAE J1587 All by the American Society of Automotive Engineers （Society of Automotive Engineers） It is a heavy motor vehicle （ truck , Passenger car , Ship, etc ） Established communication standards . among SAE J1708 The agreement OSI（Open System Interconnection） In the seven layer model, there are specifications on the physical link layer , and SAE J1587 A protocol is a specification on the application layer . The following is an introduction to these two protocol applications .

Two ,SAE J1708 agreement

1, Physical layer properties

Idle level voltage ：5V （ General gasoline vehicle 12V, diesel engined car 24V, Some cars 5V）*
Default positive and negative logic ： Positive logic ( That is, the default high-level state is 1)
Transceiver ：RS485 Transceiver
Level characteristics ：

Level characteristic ：J1708 The bus consists of two cables （A and B） form , The maximum length of twisted pair 40 rice . The voltage level on the bus is determined by the line “A” and “B” Determine the potential difference between .
Logic high level （1）： Express A Point higher B Point at least 200 mV;
Logic low level （0）： Express A The point is at least lower than B spot 200 mV;
The power supply voltage of the transceiver should be +6V To -6V（ Relative to the same ground , For all equipment ）.

2, Link layer properties

The agreement stipulates some information as follows , The corresponding attribute information value can be clearly understood in combination with the picture ;

Baud rate ：9600bps ( That is, send a single bit The time required is about 104.16us)
Communication pin ：12/13 ( Double access ,b These two pins are not fixed )
data format ：1 +8 +1 （1 Start bit position + 8 Data bit position + 1 End bit position ）

Verification method ：SAE J1708 The protocol adopts the verification method that the sum verification is zero , It has guaranteed to provide reliable data transmission services to the application layer . The calculation is as follows ：
set up x = All the data （MID+PID+DATA+CS） Sum after adding ,
if x & 0xFF = 0 , Then the data is correct , Otherwise, the data verification fails ;
give an example ： The order is “AC 00 F3 61”, be X = AC + 00 + F3 + 61 = 0x100;
check ：x&0xFF = 00, The current data is correct ;

3, visit SAE J1708 The Internet

When the device requests access to the bus , A process must be executed to invoke the arbitration mechanism . The specific call steps are as follows ：
1, Wait for the bus to idle ;
2, After leisure , The priority time delay required to wait for the requested command has begun to time ;
3, Make sure the bus is still idle , If you're not free , Return from step 1;
4, Transmission of equipment on the bus MID
5, Receive transmitted MID, And determine the sent MID With the received MID matching .
6, If MID The match is successful , We will declare on the bus , And send data packets to the module ;
7, If the match fails , We will abandon arbitration , Continue with step 8
8, If this is the first collision （ The above precautions ）, Then go back to step 1;
9, Wait for the bus to idle ;
10, Waiting for a pseudo-random number of bits （0-7 Between ）;
11, Continue with step 3
Bus idle diagram ：

Bus access time formula ：

Two ,SAE J1587 agreement

1, Message properties
What the application layer passes is not bit Stream or byte , It's news . stay SAE J1587 Agreement , The format of the message is as follows ：

> MID + (One or More Parameters) + Checksum

MID Detailed explanation ：
MID The full name is Message Identification, Message ID , The number used to identify the system module ;
for example MID = 0x80, Express Engine #1, namely 0x80 Indicates engine 1 Of ID;
SAE The association has designated most of the messages ID Store in MID In the list , It can be found in the appendix 1 Check out the section , Or download the public agreement document to view the full version ;

PID Detailed explanation ：
PID The full name is Parameter Identification, That is, the parameter represents , Used when the module performs functions , Number identifying the function to be performed ;
for example ：PID = 0x80, identification Component-specific request, namely 0x80 Requests that identify specific components ;
SAE The association has designated most of the messages ID Store in PID In the list , It can be found in the appendix 1 Check out the section , Or download the public agreement document to view the full version ;

0x00<= PID <=0x7F  Followed by a byte of data 
0x80 <=PID <=0xBF  Followed by two bytes of data 
0xC0<= PID<=0XFD  Followed by any byte of data 

Checksum Detailed explanation ：
checksum That is verification , Follow the verification method of zero cumulative sum mentioned above ;

2, Application layer properties

Single frame transmission ： According to the agreement ,SAE J1587 The maximum number of bytes transmitted in a single frame of the protocol is 21 Bytes ;
The single frame message is as follows ：
Req:AC 80 ED 80 66
Ans:80 ED 11 56 50 20 20 20 30 38 37 36 33 39 20 20 20 20 20 20 7B

Multi frame transmission ： According to the agreement ,SAE J1587 The maximum number of bytes for multi frame transmission of the protocol is 16*21 = 336 Bytes ;
Req:AC 80 EA BB 69
Ans:BB C0 11 EA 20 24 32 30 39 35 32 32 33 35 50 30 31 2A 32 30 A8
Ans:BB C0 11 EA 21 39 30 32 36 36 32 50 30 31 2A 32 30 39 30 32 58
Ans:BB C0 09 EA 22 35 39 31 50 30 31 2A 31

2, Application instance
give an example ：
Req:AC 80 EA BB 69
Ans:BB C0 11 EA 20 24 32 30 39 35 32 32 33 35 50 30 31 2A 32 30 A8
Ans:BB C0 11 EA 21 39 30 32 36 36 32 50 30 31 2A 32 30 39 30 32 58
Ans:BB C0 09 EA 22 35 39 31 50 30 31 2A 31

Req Command specification ：
AC ：MID = 0xAC, Indicates an off board diagnostic request
80 : PID = 0x80, Represents a special component request , find SAE J1587 The agreement document corresponds to PID The explanation is shown in the figure below ：
EA ： find pid list , Inside pid = 0xea Means Software Identification 80 : MID = 80, Indicates engine
BB: MID =0xBB, Represents the special component of the request ID by 0xBB
69: The value calculated according to the formula with zero checksum

Empathy , available Ans The command is parsed as follows ：
C0 : Multi segment parameters （Multisection Parameter）, Send multi frame flag ;
11 ： Indicates the number of all bytes except the check bytes ; There is 0x11 Data bytes
30 ： This byte is divided into four high bits and four low bits ,3 Means a total of （3+1） Segment data ,0 It means that this is the 0 paragraph
A8 ： The check byte of this data , This result is obtained according to the above verification method ;

appendix 1 - MID list ：

appendix 2 - PID list ：