Figure 6-2. Stand-Alone; Figure 6-3. Null Modem Location; Table 6-1. Typical Null Modem Pinout - Sierra Wireless DART 200 CDPD Modem User Manual

DART 200 CDPD Modem User's Guide
Sierra Wireless, Inc.
Figure 6-2. Stand-alone RS-232
DTE
RS-232
Computer
The problem with this occurs when modems are inserted between the
DTE and a remote device with a DCE interface. The modem expects to
communicate with a DTE interface. Since this is not the case at the
remote end, where the DCE modem is trying to talk to a DCE device, a
communications failure results. The solution for this situation is a null
modem connector. A typical null modem pin out is shown in Table 6-1.

Table 6-1. Typical null modem pinout

Female
DB9
1 DCD
2 RXD
3 TXD
4 DTR
5 SGD
6 DSR
7 RTS
8 CTS
9 ---
The null modem adapter corrects the mismatch between the modem and
a terminating device having a DCE interface. The proper location for the
null modem is shown in Figure 6-3.

Figure 6-3. Null modem location

DTE
RS-232
Computer
DCE
Terminal
Male
DB9
4
3
2
6 and 1
5
4
8
7
Open
DCE DCE
Phone
Modem
Modem
Line
6 Device Attachment
DCE
RS-232
Terminal
Null modem
needed here
6-2