Sierra Wireless DART 200 CDPD Modem - Manual (Page 88)

DART 200 CDPD Modem User’s Guide 6 Device Attachment
Sierra Wireless, Inc. 6-2
Figure 6-2. Stand-alone RS-232
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 Male
DB9
1 DCD 4
2 RXD 3
3 TXD 2
4 DTR 6 and 1
5 SGD 5
6 DSR 4
7 RTS 8
8 CTS 7
9 --- Open
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
Computer
DTE
Terminal RS-232
DCE
Computer
DTE
Terminal RS-232
Modem Modem
RS-232
DCE
Phone
Line
DCE DCE
Null modem
needed here