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Table 13- 75 Data types for the parameters

Parameter and type

Modbus_Master communication rules

● MB_COMM_LOAD must be executed to configure a port before a Modbus_Master

instruction can communicate with that port.

● If a port is to be used to initiate Modbus master requests, that port should not be used by

MB_SLAVE. One or more instances of Modbus_Master execution can be used with that

port, but all Modbus_Master execution must use the same Modbus_Master instance DB

for that port.

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

communication process. Your program must poll the Modbus_Master instruction for

transmit and receive complete conditions.

S7-1200 Programmable controller

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

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 (non-optimized 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 Modbus_Master operation in progress

1 – Modbus_Master operation in progress

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

nated with an error. The error code value at the STATUS parameter is

valid only during the single scan where ERROR = TRUE.

Execution condition code

Communication processor and Modbus TCP

13.5 Modbus communication

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