6-10 The Message Format
The quantity indicates how many consecutive registers were written to.
A CRC-16 value is generated from a calculation using the values of the address, function code,
starting register number, and quantity value of the message. The procedure for calculating a CRC-16
value is described at the end of this chapter. When the master receives the response message, it
calculates a CRC-16 value and compares it to the one in the CRC-16 field of the response
message. If these two CRC-16 values are the same the master has received the proper response
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 10h. 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 to be written to an invalid register. 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
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 response message. If these
two CRC-16 values are the same the master has received the proper response message.
CDh
C1h
CRC-16
LOWER
UPPER
01h
SLAVE ADDRESS
80h + FUNC. CODE
90h
02h
ERROR CODE
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