YASKAWA GPD 515/G5 Modbus RTU Technical Manual page 28

A normal response message for the loop-back test should be identical to the command 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. How the response message is handled by the master depends on what
master is used. When the master receives the fault response message it should calculate a
CRC-16 value and compare 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.
6-8 The Message Format
Normal Response Message
SLAVE ADDRESS
01h
FUNCTION CODE
TEST
UPPER
CODE
LOWER
UPPER
A5h
DATA
LOWER
LOWER DAh
CRC-16
UPPER
8Dh
Fault Response Message
SLAVE ADDRESS
01h
80h + FUNC. CODE 88h
ERROR CODE
01h
87h
LOWER
CRC-16
UPPER
C0h
08h
00h
00h
37h