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CT MODBUS RTU Technical Data
E300 Design Guide 483
Issue Number: 1
Master request
Slave response
11.6 Exceptions
The slave will respond with an exception response if an error is detected
in the master request. If a message is corrupted and the frame is not
received or the CRC fails then the slave will not issue an exception. In
this case, the master device will time out. If a write multiple (FC16 or
FC23) request exceeds the slave maximum buffer size then the slave
will discard the message. No exception will be transmitted in this case
and the master will time out.
Exception message format
The slave exception message has the following format:
Exception codes
The following exception codes are supported:
11.7 CRC
The CRC is a 16 bit cyclic redundancy check using the standard CRC-16
polynomial x16 + x15 + x2 + 1. The 16 bit CRC is appended to the
message and transmitted LSB first.
The CRC is calculated on ALL the bytes in the frame.
Byte Value Description
0 0x01 Slave destination node address
1 0x17 Function code 0x17
20x40
Start register Pr 00.057
16384 + (100 x 0) + 57 – 1) = 16440 = 0x4038
30x38
4 0x00 Number of 16 bit registers MSB
5 0x02 Number of 16 bit registers LSB
60x40
Start register Pr 00.054
16384 + (100 x 0) + 54 – 1) = 16437 = 0x4035
70x35
0x00 Number of 16 bit registers to write MSB
0x02 Number of 16 bit registers to write LSB
0x04 Length of register data to write (bytes)
8-11
0x00 0x00
0x00 0xC8
Register data 0
16 0x6B CRC LSB
17 0x61 CRC MSB
Byte Value Description
0 0x01 Slave destination node address
1 0x17 Function code 0x10
2 0x04 Length of register data in read block (bytes)
3-6
0x00 0x00
0x00 0x00
Register data 0
7 CRC MSB
8 CRC MSB
Byte Description
0 Slave source node address
1
Original function code with bit7 set (e.g. FC 0x03 will be
returned as 0x83)
2 Exception code
3 CRC LSB
4 CRC MSB
Byte Description
1 Function code not supported
2
Register address out of range, or request to read too many
registers
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