Siemens Simatic S7-200 System Manual page 168

Network Communications and the S7-200 CPU
Using a Modem with a PC/PPI Cable
You can use the PC/PPI cable to connect the RS-232 communication port of a modem to an
S7-200 CPU. Modems normally use the RS-232 control signals (such as RTS, CTS, and
DTR) to allow a PC to control the modem. The PC/PPI cable does not use any of these
signals, so when you use a modem with a PC/PPI cable, the modem must be configured to
operate without these signals. As a minimum, you must configure the modem to ignore RTS
and DTR. Consult the operator manual supplied with the modem to determine the
commands required to configure the modem.
When connecting a modem to a PC/PPI cable, you must use a null modem adapter between
the modem and the RS-232 port of the PC/PPI cable. Modems are classified as Data
Communications Equipment (DCE). The RS-232 port of the PC/PPI cable is also classified
as DCE. When you connect two devices of the same class (both DCE), the transmit data and
receive data pins must be swapped. A null modem adapter swaps the transmit and receive
lines. See Figure 9-7 for a typical setup and the pin assignment for a null modem adapter.
When using STEP 7-Micro/WIN with a modem, you must use a full-duplex modem which
supports 11 bit characters. See Section 3.3 for more information about using
STEP 7-Micro/WIN with a modem. When using a modem with a user-defined Freeport
protocol, you can use any modem which supports the character size of the protocol.
Figure 9-7
9-12
PC/PPI
cable
S7-200
Modem with Null Modem Adapter
RS-232
Modem
Null modem adapter
9 pin 25 pin
2 2 TD
3 3 RD
4 RTS
5 CTS
6 DSR
8 DCD
20 DTR
5 7 GND
S7-200 Programmable Controller System Manual
C79000-G7076-C230-02