Siemens SIMATIC S7-1200 System Manual page 1144

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Communication processor and Modbus TCP

13.3 Point-to-point (PtP) communication

Network 3: Enable the SEND_PTP instruction when the M20.0 flag is set. Also use this flag

to set the REQ input to TRUE for one scan. The REQ input tells the SEND_PTP instruction

that a new request is to be transmitted. The REQ input must only be set to TRUE for one

execution of SEND_PTP. The SEND_PTP instruction is executed every scan until the

transmit completes. The transmit is complete when the last byte of the message has been

transmitted from the CM 1241. When the transmit is complete, the DONE output (Tag_5 at

M10.0) is set TRUE for one execution of SEND_PTP.

Network 4: monitor the DONE output of SEND_PTP and reset the transmit flag (Tag_8 at

M20.0) when the transmit operation is complete. When the transmit flag is reset, the

RCV_PTP instruction in Network 1 is enabled to receive the next message.

S7-1200 Programmable controller

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