Siemens SIMATIC S7 System Manual page 202

Program instructions
7.3 Communication
Transmit data
The Transmit instruction lets you send a buffer of one or more characters, up to a maximum
of 255. The following figure shows the format of the Transmit buffer.
①
②
If an interrupt routine is attached to the transmit complete event, the CPU generates an
interrupt (interrupt event 9 for port 0 and interrupt event 26 for port 1) after the last character
of the buffer is sent.
You can transmit without using interrupts (for example, sending a message to a printer) by
monitoring SM4.5 (port 0) or SM4.6 (port 1) to signal when transmission is complete.
You can use the Transmit instruction to generate a BREAK condition by setting the number
of characters to zero and then executing the Transmit instruction. This generates a BREAK
condition on the line for 16-bit times at the current baud rate. Transmitting a BREAK is
handled in the same manner as transmitting any other message, in that a Transmit interrupt
is generated when the BREAK is complete and SM4.5 or SM4.6 signals the current status of
the Transmit operation.
202
Number of bytes to transmit
Characters of the message
System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART