Siemens SIMATIC S7-1200 - 13.3.3.2 Configuring transmit (send) parameters

Communication processor and Modbus TCP
13.3 Point-to-point (PtP) communication
S7-1200 Programmable controller
1096 System Manual, V4.2, 09/2016, A5E02486680-AK
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 1093), the CM aborts the transmission and returns an error to the user
program.
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.
Before the CPU can engage in PtP Freeport 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 device.
13.3.3.2
Configuring transmit (send) parameters
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
interface.
You can also dynamically configure or change the transmit message parameters from the
user program by using the Send_Config (Page 1112) instruction.
Note
Parameter values set from the Send_Config instruction in the user program override the
"Transmit message configuration" propert
ies. Note that the CPU does not retain parameters
set from the Send_Config instruction in the event of power down.