20
MRX-900 Operating Manual: RS-232 Interface and Cables
“Crossover” / “Null”
DB9-to-DB9
RS-232 Cable
9-pin
9-pin
1,6
4
2
3
3
2
4
1,6
5
5
7
8
8
7
9 (n/c)
(n/c) 9
“Crossover” / “Null”
DB9-to-DB25
RS-232 Cable
9-pin
25-pin
1,6
20
2
2
3
3
4
6,8
5
7
7
5
8
4
9 (n/c)
(n/c) 9
“Crossover” / “Null”
DB25-to-DB25
RS-232 Cable
25-pin
25-pin
1
1
2
3
3
2
4
5
5
4
6,8
20
7
7
20
6,8
Note that in all cases, the DSR and
DCD lines are tied together (lines 1
and 6 for DB9; 6 and 8 for DB25).
DCE DB9 Loopback Plug
9-pin male
connector
1 (n/c)
2
3
4
5 (n/c)
6
7
8
9 (n/c)
C.2 Crossover / Null Modem Cables
Typically, establishing a DTE-to-DCE connection is achieved using a
straight-through cable as described earlier. In some instances, generally
when a direct DTE-to-DTE or DCE-to-DCE connection is desired, a
crossover cable (also known as rollover, null-modem, or modem-eliminator
cable) is used. Using a straight-through cable will not work
In a DTE-to-DTE connection, the crossover cable essentially functions by
deceiving each DTE into “thinking” that it is connected to a modem.
Similarly, a DCE connected to a DCE “believes” it is connected to a DTE.
The data transmission is handled such that transmitted data (TX) from one
device is driven to the receive data line (RX) of the other. RTS is associated
with CTS. Since a DCE in a typical DTE-to-DCE connection usually must
indicate its own online status (via DSR) as well as its connection to another
DCE (via DCD), these are relayed together as a ready-state indicator from the
DTE via DTR.
Wiring diagrams for the most common cables of this type are provided for
your reference.
C.3 Loopback Plug
Occasionally, you may wish to perform diagnostic or benchmark tests on the
data transmission between two MRX-900s. A common test is the loopback,
whereby an MRX-900, connected to a DTE, communicates with another
MRX-900, whose serial port has been fitted with a DCE loopback plug. By
so doing, all data sent by the DTE should be wirelessly transmitted to the
remote MRX-900, which echoes back the data to the originator, namely the
DTE. Essentially the same data is transmitted twice: once to the remote
modem, and the second time as it is re-transmitted back from the remote
modem. Such tests are useful for testing the performance of the wireless link,
including throughput, bit error rate, packet loss, and a number of other
measures.
The wiring for a basic 9-pin DCE loopback plug is provided, which can be
constructed simply by tying together pins 2 and 3, as well as 7 and 8, on a
male DB9 connector.
Testing that a DTE serial port is functional is often achieved easily by using a
DTE loopback plug, which can be constructed similarly to the DCE loopback
plug for a 9-pin connector. The gender is often different (use a female
connector for use on a common PC), but the same pins are tied together: 2
with 3, and 7 with 8. Additionally, pins 4 and 6 should be tied together. (For
a 25-pin DTE loopback plug, connect pins 2 with 3, 4 with 5, and 6 with 20.)
Summary of Contents for MRX-900
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