Company Confidential
29
Raveon Technologies Corp.
Appendix A. Null Modem Cables
Null modem without handshaking
How to use the handshaking lines in a null modem configuration? The simplest
way is to don't use them at all. In that situation, only the data lines and signal
ground are cross connected in the null modem communication cable. All other
pins have no connection. An example of such a null modem cable without
handshaking can be seen in the figure below.
Simple null modem without handshaking
(DB-9 Female shown. Same wiring for male-to-male cable)
Connector 1
Connector 2
Function
2
3
Rx
Tx
3
2
Tx
Rx
5
5
Signal ground
Compatibility issues
There is a problem, if either of the two devices checks the
DSR
or
CD
inputs.
These signals normally define the ability of the other side to communicate. As
they are not connected, their signal level will never go high. This might cause
a problem.
The same holds for the
RTS
/
CTS
handshaking sequence. If the software on
both sides is well structured, the
RTS
output is set high and then a waiting
cycle is started until a ready signal is received on the
CTS
line. This causes
the software to hang because no physical connection is present to either
CTS
line to make this possible. The only type of communication which is allowed
on such a null modem line is data-only traffic on the cross connected
Rx
/
Tx
lines.
Null modem with full handshaking
In this null modem cable, seven wires are present. The cable is shown in the
following figure.
Null modem with full handshaking
