Requirements For Using Modbus Instructions - Siemens SIMATIC S7 System Manual

Hide thumbs Also See for SIMATIC S7:

Table of Contents

Requirements for using Modbus instructions

Modbus RTU master protocol

Modbus master instructions use the following resources from the CPU:

● MBUS_CTRL / MB_CTRL2 (Page 481) execution initializes the Modbus master protocol

and dedicates the assigned CPU port (0 or 1) for Modbus master communication.

When you use a CPU port for Modbus communications, you cannot use it for any other

purpose, including communication with an HMI.

● Modbus master instructions affect all of the SM locations associated with Freeport

communications on the port assigned by the MBUS_CTRL / MB_CTRL2 instruction.

● Modbus master instructions use interrupts for some functions. These interrupts must not

be disabled by the user program.

● Modbus master instructions program size

– 3 subroutines and 1 interrupt routine

– 1942 bytes of program space for two master instructions and support routines

– Variables for Modbus master instructions require a 286 byte block of V memory. You

S7-200 SMART

System Manual, V2.3, 07/2017, A5E03822230-AF

must assign the starting address for this block using the Library Memory command in

STEP 7-Micro/WIN SMART. This command is available from the shortcut memory of

the Library node under the Program Block node in the project tree, or from the

Libraries section of the File menu ribbon strip.

To change the CPU communication port from Modbus back to PPI so that you can

communicate with an HMI device, set the mode parameter of the MBUS_CTRL /

MB_CTRL2 instruction to a zero (0).

9.3 Modbus RTU library

Show Quick Links

Table of Contents

Simatic s7-200 smart