User's Manual

Company Confidential 44 Raveon Technologies Corp.
10.4. 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
(DB-9 Female shown. Same wiring for male-to-male cable)
Connector 1 Connector 2 Function
2 3 Rx Tx
3 2 Tx Rx
4 6 DTR DSR
5 5 Signal ground
6 4 DSR DTR
7 8 RTS CTS
8 7 CTS RTS
10.5. Compatibility issues
The null modem cable with full handshaking does not permit the older way of flow
control to take place. The main incompatibility is the cross connection of the RTS
and CTS pins. Originally, these pins are used for a question/answer type of flow
control. When the full handshaking null modem cable is used, there is no request
anymore. The lines are purely used for telling the other side if communication is
possible.
The main advantage of this cable is, that there are two signaling lines in each
direction. Both the RTS and DTR outputs can be used to send flow control
information to the other device. This makes it possible to achieve high
communication speeds with this type of null modem cable, provided that the
system has been designed for it.