Siemens Simatic S7-200 System Manual page 347

MBUS_SLAVE Instruction
The MBUS_SLAVE instruction is used to service a request from the
Modbus master and must be executed every scan to allow it to
check for and respond to Modbus requests.
The instruction is executed on each scan when the EN input is on.
The MBUS_SLAVE instruction has no input parameters.
The Done output is on when the MBUS_SLAVE instruction
responds to a Modbus request. The Done output is turned off if
there was no request serviced.
The Error output contains the result of executing the instruction.
This output is only valid if Done is on. If Done is off, the error
parameter is not changed. Table 12-5 defines the error conditions
that could result from executing the instruction.
Table 12-4 Parameters for the MBUS_SLAVE Instruction
Parameter Data Type
Done BOOL
Error BYTE
Table 12-5 Modbus Slave Protocol Execution Error Codes
Error Codes Description
0 No Error
1 Memory range error
2 Illegal baud rate or parity
3 Illegal slave address
4 Illegal value for Modbus parameter
5 Holding registers overlap Modbus Slave symbols
6 Receive parity error
7 Receive CRC error
8 Illegal function request/function not supported
9 Illegal memory address in request
10 Slave function not enabled
Using the Modbus Protocol Library
Operands
I, Q, M, S, SM, T, C, V, L
VB, IB, QB, MB, SB, SMB, LB, AC, *VD, *AC, *LD
Chapter 12
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