Siemens SIMATIC S7 System Manual page 469

If you use the RS232/PPI Multi-Master cable in a system which uses Freeport
communications, the program in the S7-200 SMART CPU must comprehend the turnaround
time for the following situations:
● The CPU responds to messages transmitted by the RS232 device.
After the CPU receives a request message from the RS232 device, the CPU must delay
the transmission of a response message for a period of time greater than or equal to the
turnaround time of the cable.
● The RS232 device responds to messages transmitted from the CPU.
After the CPU receives a response message from the RS232 device, the CPU must delay
the transmission of the next request message for a period of time greater than or equal to
the turnaround time of the cable.
In both situations, the delay allows the RS232/PPI Multi-Master cable sufficient time to
switch from Transmit mode to Receive mode so that data can be transmitted from the RS485
port to the RS232 port.
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
Communication
8.6 RS485
469