Specifications

RV-M7 Technical Manual
60
15.2 Null modem without handshaking
Sometimes, a “Null Modem” cable may be required to connect the M7 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
15.3 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.