Requirements For Using Modbus Instructions - Siemens SIMATIC S7 System Manual

9.3.1.2

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.
Note
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).
Libraries
9.3 Modbus RTU library
477