Siemens SIMATIC S7-1200 System Manual page 1208

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Table of Contents

Communication processor and Modbus TCP

13.5 Modbus communication

Table 13- 84 Diagnostic functions

S7-1200 Modbus_Slave Modbus diagnostic functions

Sub-function

The Modbus_Slave instruction supports broadcast write requests from any Modbus master

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

code "0x8188" for function codes not supported in broadcast.

Modbus_Slave communication rules

● Modbus_Comm_Load must be executed to configure a port, before a Modbus_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 Modbus_Master instruction.

● Only one instance of Modbus_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 Modbus_Slave instruction for transmit and receive complete conditions.

● The Modbus_Slaveinstruction 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 Modbus_Slave every scan from a program cycle OB. Executing Modbus_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.

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

word of data that is received.

Clear communication event counter: The Modbus_Slave will clear out the communica-

tion event counter that is used for Modbus function 11.

Get communication event counter: The Modbus_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).

S7-1200 Programmable controller

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