Interpreters Document

Typical Interpreter Line: (all Action and Goto fields are hex numbers) 
Step 
Op-Code 
Action Fields
Goto Fields 
10 
78 
20 00 03 00 
78 11 10 0D FF 99 00 00 00 00 
CameraLoops.com

---


1993-2011 General Motors Company, GM Customer Care & Aftersales. All Rights Reserved.
Page
140 
81 Op-Code: SR 81 to Start Communications
 
Supported By:
KWP2000 Interpreter 2
 
 
Description: 
The 81 Op-Code is used to start communications with an ECU. Communications will remain established while
activity exists on the link. If the link is inactive for five seconds the ECU will revert back to a waiting for 
communications state. There are no negative responses allowed for the Start Communication request.
Action Fields: 
AC0: $00 - Not used
AC1: $00 - Not used
AC2: $00 - Not used
AC3: Exceptions: $00 - Not used
Pseudo Code: 
XX 81 AC0 AC1 AC2 AC3 G0-G1 G2-G3 G4-G5 G6-G7 G8-G9 
Use KWP2000 Normal Response processing 
Wait for W5min idle time
Send wake up pattern to the ECU 
Build Start Communication Request as indicated in the table below 
Request 
Byte
Value 
Byte Description 
1 
81 
Format byte for physical addressing and length 
2 
## 
Global target byte 
3 
## 
Global source byte 
4 
81 
Start communication request service ID
5 
CS 
1 byte service request checksum
Calculate checksum and copy into the service request (byte 5) 
Send Start Communication Request and get the response (up to 10 Bytes) 
Goto KWP2000 Response Processing
Set addressing mode and use of length information based on two returned key bytes 
Look for Start Communication positive response service ID ($C1) in goto fields 
If found
Goto appropriate step
Else 
Goto step indicated by FF goto field 
Endif