The Message Format 6-7
Normal Response Message
A normal response message for the loop-back test should be identical to the command message.
Fault Response Message
The fault response message contains the same slave address as the command message,
indicating to the master which slave is responding.
The function code of a fault response message is actually a value of 80h plus the original function
code of 08h. This indicates to the master that the message is a fault response message, instead of
a normal response message.
The error code indicates where the error occurred in the command message. A complete listing of
the error codes is shown in Chapter 8, Troubleshooting and Error Codes.
A CRC-16 value is generated from a calculation using the values of the address, function code, and
data sections of the message. The procedure for calculating a CRC-16 value is described at the end
of this chapter. When the master receives the fault response message it calculates a CRC-16 value
and compares it to the one in the CRC-16 field of the fault response message. If these two CRC-16
values are the same the master has received the proper fault response message.
01h
SLAVE ADDRESS
80h + FUNC. CODE
88h
01h
87h
C0h
CRC-16
LOWER
UPPER
ERROR CODE
DATA
01h
SLAVE ADDRESS
FUNCTION CODE
08h
00h
00h
A5h
37h
DAh
8Dh
TEST
CODE
CRC-16
UPPER
LOWER
UPPER
LOWER
LOWER
UPPER