Siemens Simatic S7-200 System Manual page 235

Using the PC/PPI Cable and Freeport Mode with RS-232 Devices
You can use the PC/PPI cable and the Freeport communications functions to connect the S7-200 CPUs to
many devices that are compatible with the RS-232 standard.
The PC/PPI cable is in Transmit mode when data is transmitted from the RS-232 port to the RS-485 port.
The cable is in Receive mode when it is idle or is transmitting data from the RS-485 port to the RS-232
port. The cable changes from Receive to Transmit mode immediately when it detects characters on the
RS-232 transmit line.
The PC/PPI cable supports baud rates between 1200 baud and 115.2 kbaud. Use the DIP switches on the
housing of the PC/PPI cable to configure the cable for the correct baud rate. Table 7-9 shows the baud
rates and switch positions.
The cable switches back to Receive mode
when the RS-232 transmit line is in the idle
state for a period of time defined as the
turnaround time of the cable. The baud rate
selection of the cable determines the
turnaround time, as shown in Table 7-9.
If you are using the PC/PPI cable in a system
where Freeport communications are used, the
program in the S7-200 must comprehend the
turnaround time for the following situations:
-
The S7-200 responds to messages transmitted by the RS-232 device.
After the S7-200 receives a request message from the RS-232 device, the S7-200 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 RS-232 device responds to messages transmitted from the S7-200.
After the S7-200 receives a response message from the RS-232 device, the S7-200 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 PC/PPI cable sufficient time to switch from Transmit mode to
Receive mode so that data can be transmitted from the RS-485 port to the RS-232 port.
Communicating over a Network
Table 7-9 Turnaround Time and Settings
Baud Rate Turnaround Time
38400 to 115200
19200
9600
4800
2400
1200 14.0 ms
Chapter 7
Settings (1 = Up)
0.5 ms 000
1.0 ms 001
2.0 ms 010
4.0 ms 011
7.0 ms 100
101
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