Programming The Step 7 Program - Siemens SIMATIC S7-1200 System Manual

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Case 5: RS485: No bias (external bias)

● Mode of operation: RS485

● Receive line initial state: No bias (external bias required)

Programming the STEP 7 program

The example program uses a global data block for the communication buffer, a RCV_PTP

instruction (Page 1229) to receive data from the terminal emulator, and a SEND_PTP

instruction (Page 1226) to echo the buffer back to the terminal emulator. To program the

example, add the data block configuration and Main program block OB 1 as described

Global data block "Comm_Buffer": Create a global data block (DB) and name it

"Comm_Buffer". Create one value in the data block called "buffer" with a data type of "array

[0 .. 99] of byte".

Network 1: Enable the RCV_PTP instruction whenever SEND_PTP is not active. Tag_8 at

MW20.0 indicates when sending is complete in Network 4, and when the communication

module is thus ready to receive a message.

Network 2: Use the NDR value (Tag_1 at M0.0) set by the RCV_PTP instruction to make a

copy of the number of bytes received and to set a flag (Tag_8 at M20.0) to trigger the

SEND_PTP instruction.

S7-1200 Programmable controller

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

Communication processor and Modbus TCP

13.3 Point-to-point (PtP) communication