The "Claim To Fame" for the Hayes modems was, as John Levine pointed out, that it could be used with only a three wire connection. This may seem like a solution in search of a problem today, but return with me now to those thrilling days of yesteryear, prior to the Borg -
- There was no agreement on what connectors to use: my Heath H89 had both female and male 25 pin connectors for the DTE ports.
- Retailers would sell _anything_ that looked like a computer cable, no matter what the connector sex, the wire, or the pinout.
- Operating Systems did _NOT_ have complete control of the serial ports. CP/M required drivers that were written by OEM's, or even by end-users like me, and everyone was in an incredible hurry to get product to market, so "DSR" and "CD" leads were often ignored. Hell, it was hard enough to get the speed right, with some control programs requiring manual setup for the modem speed since they had no "auto detect" capability.
- Software vendors advertised "technical support" very heavily, but what they provided was a long list of excuses for doing nothing: if the modem lights blinked, they would tell you it was a modem problem and refer you back to your modem vendor.
The Hayes modems worked if you could send them data and receive data, and they succeeded for that reason. It wasn't until the IBM PC took over the "baseline" position in hardware comparisons that a semblance of order was introduced, with DTE ports having male connectors and DCE using female, with machine-to-machine serial connections requiring null-modem cables, and with each OS properly handling supervisory lines.
As with the Centronics interface for printers, the Hayes command set became the de facto standard and is used to this day.
Bill Horne (Filter QRM for direct replies)