Modbus Tcp Server; Data Access Using Modbus Tcp Protocol; Exception Code; Modbus Requests - Bender COMTRAXX CP9 Series Manual

Modbus TCP server

4. Modbus TCP server

4.1 Data access using Modbus TCP protocol

Requests are sent to the Modbus TCP server of the CP9xx using function code FC4 (read input register). The ser-
ver generates a function-related response and sends it to the Modbus client.

4.1.1 Exception code

If a request cannot be answered for whatever reason, the server sends a so-called exception code with which
possible faults can be narrowed down.
Exception code
0x01
0x02
0x03
0x04
0x05
0x06
0x08
0x0A
0x0B

4.1.2 Modbus requests

The required words of the process image can be read from the input registers of the CP9xx using the function
code FC4. For this purpose, the start address and the number of the registers to be read out have to be entered.
Example:
The words 0 and 1 are to be read from the input registers 0x100 and 0x101.
Byte
Byte 0, 1
Byte 2, 3
Byte 4, 5
Byte 6
Byte 7
Byte 8, 9
Byte 10, 11

4.1.3 Modbus responses

The responses consist of 2 bytes per register. The MSB is the first byte.
Byte
...
Byte 7
Byte 8
Byte 9, 10
Byte 11, 12

4.1.4 Structure of the exception code

Byte
...
Byte 7
Byte 8
CP9xx_D00349_07_M_XXEN/01.2020
Description
Impermissible function
Impermissible data access
Impermissible data value
Slave device error
Acknowledgement of receipt (answer will be time-delayed)
Request not accepted (repeat request, if necessary)
Memory: Parity Error
Gateway path not available
Gateway error
Name
Transaction identifier
Protocol identifier
Length field
Unit identifier
Modbus function code
Device address (BMS int * 0x100)
Number of words
Name
...
MODBUS function code
Byte count
Value register 0
Value register 1
Name
...
MODBUS function code
Exception code
Example
0x0000
0x0000
0x0006
Subsystem address assign-
ment. 0x01 (corresponds to
the subsystem address 1)
0x04
Device address assignment.
0x0100 (corresponds to the
device address 1)
0x0002
Example
...
0x04
0x04
0x1234 (fictitious value)
0x2345 (fictitious value)
Example
...
0x84
0x01 or 0x02
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