Siemens SIMATIC S7-200 System Manual page 241

Using the RS-232/PPI Multi-Master Cable and Freeport Mode with
RS-232 Devices
You can use the RS-232/PPI Multi-Master cable and the Freeport communications functions to
connect the S7-200 CPUs to many devices that are compatible with the RS-232 standard. The
cable must be set to PPI/Freeport mode (switch 5 = 0) for Freeport operation. Switch 6 selects
either Local mode (DCE) (switch 6 = 0), or Remote mode (DTE) (switch 6 = 1).
The RS-232/PPI Multi-Master 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 RS-232/PPI Multi-Master cable supports baud rates between 1200 baud and 115.2 kbaud.
Use the DIP switches on the housing of the RS-232/PPI Multi-Master cable to configure the cable
for the correct baud rate. Table 7-11 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 7-11.
If you are using the RS-232/PPI
Multi-Master cable in a system where
Freeport communications is 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 RS-232/PPI Multi-Master 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 7-11 Turnaround Time and Settings
Baud Rate Turnaround Time
115200
57600
38400
19200
9600
4800
2400
1200
Chapter 7
Settings (1 =
Up)
0.15 ms 110
0.3 ms 111
0.5 ms 000
1.0 ms 001
2.0 ms 010
4.0 ms 011
7.0 ms 100
14.0 ms 101
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