User's Manual

Company Confidential 64 Raveon Technologies
14.2 Null modem without handshaking
Sometimes, a “Null Modem” cable may be required to connect the M7-UC
modem to another device. The specific connections are very dependent upon
the type of hardware and handshaking used, but the following sections should
help in configuring a null-modem cable.
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
14.3 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.