> 
> OK, here's the scoop on hardware handshaking:
> 
> In an ideal world, you have at least 8 pins on an RS-232 serial cable. The
> standard PC 9-pin serial cable includes the following pins (not in order),
> and the 25-pin cable has even more, mostly useless:
> 
> Electrical Ground
>  - completes circuit for the rest of the pins
> 
> Receive Data
>  - guess what this does
> 
> Transmit Data
>  - guess again
> 
> RTS (Request to Send) Hardware Handshaking
> CTS (Clear to Send) Hardware Handshaking
>  - These are the standard handshaking signals by which the computer and the
> peripheral tell each other to shut up. RTS is controlled by the computer
> and tells the modem (or whatever) to shut up, and CTS is controlled by the
> modem and tells the computer to shut up. (signal up = allow data flow,
> signal down = shut up)
> 
> DTR (Data Terminal Ready) Hardware Handshaking
> DSR (Data Set Ready) Hardware Handshaking
>  - This is another set of hardware handshaking signals. DTR corresponds to
> RTS, and DSR corresponds to CTS. These aren't used for their intended
> purpose, however, because RTS and CTS already exist. DSR isn't used, for
> the most part. However, DTR is used by the computer to tell the modem to
> hang up. (signal up = keep connected, signal down = hang up)
> 
> DCD (Data Carrier Detect) Hardware Handshaking
>  - This is used by modems to tell computers whether they're connected or
> not. It provides very nice hangup detection. Unfortunately, Mac modem
> cables often don't support this signal, and Mac software usually doesn't.
> (signal up = connected, signal down = not connected)

I disagree with the third sentance above. All the cables I've seen recently
DO support DCD, and I think some MacOS software supports it. Other folks
have had cables which don't, but I've only seen ones which do support it.

Also, NetBSD/mac68k definitely supports it.

> RI (Ring Indicator)
>  - This is supposed to be a ring indicator. I've never seen this used.
> 
> The pins above total 9; minus the unnecessary Ring Indicator, 8. Mac serial
> cables/ports have 8 pins; however, Mac serial ports aren't RS-232. They're
> a superset standard (RS-422 or something like that) that uses 2 additional
> pins to provide clearer communicaton over longer serial cables. Thus, two
> handshake pins can't be used.

Having the differential inputs & outputs  also makes LocalTalk work better.

> The solution currently used is to put RTS and DTR on one pin (HSKo or
> Handshake Out I think) and CTS and DSR on another (HSKi or Handshake In
> probably). DCD is renamed to GPi (General Purpose Input).
> 
> DSR and CTS happily co-exist, because DSR isn't used. However, RTS and DTR
> are both used by computers to send signals to modems, so this is a problem.

I agree. Just ignore DSR.

Take care,

Bill