Siemens S7-1200 System Manual page 750

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Communication processor and Modbus TCP

12.5 Modbus communication

Table 12- 68 Data types for the parameters

Parameter and type

0=No request

1= Request to transmit data to Modbus slave

Modbus RTU station address:

Standard addressing range (1 to 247)

Extended addressing range (1 to 65535)

The value of 0 is reserved for broadcasting a message to all Modbus

slaves. Modbus function codes 05, 06, 15 and 16 are the only function

codes supported for broadcast.

Mode Selection: Specifies the type of request (read, write, or diagnostic).

See the Modbus functions table below for details.

Starting Address in the slave: Specifies the starting address of the data to

be accessed in the Modbus slave. See the Modbus functions table below

for valid addresses.

Data Length: Specifies the number of bits or words to be accessed in this

request. See the Modbus functions table below for valid lengths.

Data Pointer: Points to the M or DB address (Standard DB type) for the

data being written or read.

The DONE bit is TRUE for one scan, after the last request was completed

with no error.

0 – No MB_MASTER operation in progress

1 – MB_MASTER operation in progress

The ERROR bit is TRUE for one scan, after the last request was

terminated with an error. The error code value at the STATUS parameter

is valid only during the single scan where ERROR = TRUE.

Execution condition code

S7-1200 Programmable controller

System Manual, 03/2014, A5E02486680-AG