Sierra Wireless DART 200 CDPD Modem manual Stand-alone RS-232, Null modem location

DART 200 CDPD Modem User’s Guide 6 Device Attachment

Figure 6-2. Stand-alone RS-232

DTE

Computer

RS-232

DCE

Terminal

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	Male
DB9	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

DTE

DCE

DCE

DCE

Computer

RS-232

Modem

Phone

Line

RS-232Terminal

Modem

Null modem needed here

Sierra Wireless, Inc. 6-2