GPD315 Modbus RTU Technical Manual TM 4325
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11
Normal Response Message
Table 4-3 Read Normal Response Message
Slave Address
02h
Function Code
03h
Number of Data Bytes
08h
Upper
17h
Starting Register
Lower
70h
Upper
17h
Next Register
Lower
70h
Upper
01h
Next Register
Lower
09h
Upper
00h
Last Register
Lower
00h
Lower
38h
CRC-16
Upper
ACh
The normal response message contains the same slave address and function code as the
command message, indicating to the master which slave is responding and to what type of
function it is responding.
The number of data bytes is the number of data bytes returned in the response message. The
number of data bytes is actually the quantity (in the command message) times 2, since there are
two bytes of data in each register.
The data section of the response message contains 8 upper and 8 lower bits of data for each
register that has been read from the drive.
A CRC-16 value is generated from a calculation using the values of the address, function code,
number of data bytes, and register 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 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.
