Null modem cable

 

 

 

 

 

The standard RS 232 is built around the presumption that at one end of the link is a DTE - data terminal equipment ( usually a computer ), and at the other end of the link is a DCE - data communications equipment ( usually a modem ).

So looking at the circuit TXD, on the DTE this is the data output; on the DCE the circuit TXD is an input looking for data from the DTE.

Similarily, on circuit CTS, the DCE originates this as a control signal, and on the DTE, this circuit is an input.

This same situation arises on all the other circuits.

Therefore a cable made up to connect a DTE to a DCE is simply wired pin to pin, ie,

 

DTE

 

DCE

TXD

to

TXD

RXD

to

RXD

RTS

to

RTS

CTS

to

CTS

DSR

to

DSR

DTR

to

DTR

etc

 

 

However, when a computer is connected to another computer, ie, a DTE to a DTE, a conflict arises, because both DTE`s wants to originate the signals on some circuits, or alternatively, both DTE`s want to receive a signal on the other circuits.

It is therefore neccessary to cross connect the circuits, so that every output on each DTE is connected to an input on the other DTE.

 

DTE 1

 

DTE 2

TXD

to

RXD

RXD

to

TXD

RTS

to

CTS

CTS

to

RTS

DSR

to

DTR

DTR

to

DSR

On this type of connection, the circuits are arranged for hardware handshaking using the circuits RTS, CTS, DSR, DTR.

Some computers may require less than this number of interconnects, and some may require more - it depends on the computers, and whether hardware or software handshaking is used.

 

© 1998 Ron Turner

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