Sierra Wireless DART 200 CDPD Modem User 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