Siemens SIMATIC S7-1200 System Manual page 1280

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Communication processor and Modbus TCP

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

The following table shows examples of Modbus address to holding register mapping that is

used for Modbus function codes 03 (read words), 06 (write word), and 16 (write words). The

actual upper limit of DB addresses is determined by the maximum work memory limit and M

memory limit, for each CPU model.

Table 13- 156 Mapping of Modbus addresses to CPU memory

Modbus Master

Table 13- 157 Diagnostic functions

S7-1200 MB_SLAVE Modbus diagnostic functions

Sub-function

The MB_SLAVE instruction supports broadcast write requests from any Modbus master as

long as the request is for accessing valid addresses. MB_SLAVE will produce error code

0x8188 for function codes not supported in broadcast.

MB_HOLD_REG parameter examples

Return query data echo test: The MB_SLAVE will echo back to a Modbus master a

word of data that is received.

Clear communication event counter: The MB_SLAVE will clear out the communication

event counter that is used for Modbus function 11.

Get communication event counter: The MB_SLAVE uses an internal communication

event counter for recording the number of successful Modbus read and write requests

that are sent to the Modbus slave. The counter does not increment on any Function 8,

Function 11, or broadcast requests. It is also not incremented on any requests that

result in a communication error (for example, parity or CRC errors).

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S7-1200 Programmable controller

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