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Modbus signal timing

Modbus_Slave must be executed periodically to receive each request from the

Modbus_Master and then respond as required. The frequency of execution for

Modbus_Slave is dependent upon the response timeout period of the Modbus_Master. This

is illustrated in the following diagram.

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 Modbus_Slave must be based upon the

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

Modbus_Slave twice within the response timeout period of the Modbus_Master.

Modbus_Slave instruction data block (DB) tags

The following table shows the public static tags stored in the Modbus_Slave instance DB that

you can use in your program:

Table 13- 85 Modbus_Slave instance DB static tags

HR_Start_Offset

Extended_Addressing

Request_Count

Slave_Message_Count

Bad_CRC_Count

Broadcast_Count

Exception_Count

Success_Count

Your program can write values to the HR_Start_Offset and Extended_Addressing tags and

control the Modbus slave operations. You can read the other tags to monitor the Modbus

S7-1200 Programmable controller

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

Communication processor and Modbus TCP

Assigns the starting address of the Modbus holding register (default =

Configures single or double-byte slave addressing:

FALSE = single byte address

TRUE = double-byte address

Total of all requests received by this slave

Number of requests received for this specific slave

Number of requests received that have a CRC error

Number of broadcast requests received

Modbus-specific errors that require an acknowledgement with a re-

turned exception to the master

Number of requests received for this specific slave that have no proto-

The MB_DB parameter of the Modbus_Comm_Load instruction must

be connected to the MB_DB parameter of the Modbus_Slave instruc-

13.5 Modbus communication

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