GPD315 Modbus RTU Technical Manual TM 4325
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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.
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