Hayes Compatible EIA RS232-C Pinouts

Conventional Hayes and Hayes Compatible modems are all wired identically. These devices are classified as DCE (Data Communications Equipment). In general they have a DB25s (Female) interface connector pinned as follows:


Pin#   FunctionSignal Direction
2Recv'd dataInput to modem
3Xmit'd dataOutput to DTE device (PC/denso)  
4Request to sendInput from DTE... PC is ready!
5Clear to sendOutput to DTE... channel ready
6Data set readyOutput to DTE... channel ready
7Signal GroundCommon
8Carrier DetectOutput to DTE... channel ready
11Alert PlusOutput to DTE...
20Data Terminal Ready   Input from DTE... PC is ready!
22Ring IndicatorOutput to DTE... line status

The RS232 standard describes two different device types; [1] devices that originate information (DTE), and [2] devices that simply pass that information along (DCE). No other decision has resulted in so much confusion. The RS232 specifications resulted in two differing wiring schemes.

In the minds of the creators, a DTE would always interface to a DCE. Thus with the paired wiring schemes, interconnection would always be via a straight, pin-to-pin cable.

Over the years hundreds of devices have been developed with RS232 communications abilities. For example both computers (and X-Rite densitometers) are classified as DTE (Data Terminal Equipment).

It has been pretty much the choice of the manufacturer as to which of the wiring schemes to adopt. Thus it is generally anything but simple to ascertain the correct wiring.

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