Siemens SIMATIC S7-1200 System Manual page 1281

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Modbus slave communication rules

● MB_COMM_LOAD must be executed to configure a port, before a MB_SLAVE instruction

can communicate through that port.

● If a port is to respond as a slave to a Modbus master, then do not program that port with

the MB_MASTER instruction.

● Only one instance of MB_SLAVE can be used with a given port, otherwise erratic

behavior may occur.

● The Modbus instructions do not use communication interrupt events to control the

communication process. Your program must control the communication process by

polling the MB_SLAVE instruction for transmit and receive complete conditions.

● The MB_SLAVE instruction must execute periodically at a rate that allows it to make a

timely response to incoming requests from a Modbus master. It is recommended that you

execute MB_SLAVE every scan from a program cycle OB. Executing MB_SLAVE from a

cyclic interrupt OB is possible, but is not recommended because of the potential for

excessive time delays in the interrupt routine to temporarily block the execution of other

interrupt routines.

Modbus signal timing

MB_SLAVE must be executed periodically to receive each request from the Modbus master

and then respond as required. The frequency of execution for MB_SLAVE is dependent

upon the response timeout period of the Modbus master. This is illustrated in the following

The response timeout period RESP_TO is the amount of time a Modbus master waits for the

start of a response from a Modbus slave. This time period is not defined by the Modbus

protocol, but is a parameter of each Modbus master. The frequency of execution (the time

between one execution and the next execution) of MB_SLAVE must be based on the

particular parameters of your Modbus master. At a minimum, you should execute

MB_SLAVE twice within the response timeout period of the Modbus master.

S7-1200 Programmable controller

System Manual, V4.2, 09/2016, A5E02486680-AK

Communication processor and Modbus TCP

13.9 Legacy Modbus RTU communication (CM/CB 1241 only)