Configuring The Transmit (Send) And Receive Parameters; Configuring Transmit (Send) Parameters - Siemens S7-1200 System Manual

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Software flow control

Software flow control uses special characters in the messages to provide flow control. You

configure Hex characters that represent XON and XOFF.

XOFF indicates that a transmission must stop. XON indicates that a transmission can

resume. XOFF and XON must not be the same character.

When the transmitting device receives an XOFF character from the receiving device, it stops

transmitting. Transmitting resumes when the transmitting device receives an XON character.

If it does not receive an XON character within the wait time that is specified in the port

configuration (Page 672), the CM aborts the transmission and returns an error to the user

Software flow control requires full-duplex communication, as the receiving partner must be

able to send XOFF to the transmitting partner while a transmission is in progress. Software

flow control is only possible with messages that contain only ASCII characters. Binary

protocols cannot utilize software flow control.

Configuring the transmit (send) and receive parameters

Before the CPU can engage in PtP communications, you must configure parameters for

transmitting (or sending) messages and receiving messages. These parameters dictate how

communications operate when messages are being transmitted to or received from a target

From the device configuration of the CPU, you configure how a communication interface

transmits data by setting the "Transmit message configuration" properties for the selected

You can also dynamically configure or change the transmit message parameters from the

user program by using the SEND_CFG (Page 688) instruction.

Parameter values set from the SEND_CFG instruction in the user program override the

"Transmit message configuration" properties. Note that the CPU does not retain parameters

set from the SEND_CFG instruction in the event of power down.

S7-1200 Programmable controller

System Manual, 03/2014, A5E02486680-AG

Communication processor and Modbus TCP

12.3 Point-to-point (PtP) communication