GPD315 Modbus RTU Technical Manual TM 4325
Page
12
Fault Response
Table 4-4 Read Fault Response Message
Slave Address
02h
Function Code
83h
Error Code
02h
Lower
30h
CRC-16
Upper
F1h
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 03h. 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. The value of 2h in
the error code field of this fault response message, indicates that the command message
requested data be read from an invalid register. A complete listing of the error codes is shown in
Chapter 6, Troubleshooting and Error Codes.
A CRC-16 value is generated from a calculation using the values of the address, function code,
and error code 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.
Loop-back Test - 08h
The loop-back test function (08h) is used for checking signal transmission between master and
slaves. The command message format is shown below.
Table 4-5 Loop Back Command Message
Slave Address
01h
Function Code
08h
Upper
00h
Test Code
Lower
00h
Upper
A5h
Lower
37h
Data
Upper
ACh
Lower
Dah
CRC-16
Upper
8Dh
Each GPD315’s slave address is set in advance by the drive parameter n106. Valid slave
addresses must be in the range of 1 to 31 decimal (1 to 1F hex). No two slaves may have the
same address. The master addresses the slave by placing the slave address in the address field
of the message. In the command message above, the slave is addressed at 1.
