From: elliott@veronica.cs.wisc.edu (James Elliott)
Subject: Re: Handshaking serial cable
Organization: U of Wisconsin CS Dept
Date: Thu, 9 Apr 1992 18:03:07 GMT

In <1992Apr9.040807.28232@dartvax.dartmouth.edu> Espen.H.Koht@dartmouth.edu (Espen H. Koht) writes:

>       Macintosh (DTE)                       Modem (DCE)
>         DIN-8 Male                          DB-25 Male
>Signal    Pin Nbr.                  Signal    Pin Nbr.
>HSKo         1        -------->      RTS          4
>HSKi         2        <--------      CTS          5
>TXD-         3        -------->      TX           2
>GND          4        ---------      signal GND   7
>RXD-         5        <--------      RXD          3
>TXD+         6  nc               |-- DSR          6
>GPi          7  nc               |-- DTR         20
>RXD+         8        ---------      signal GND   7
>Cable Shield                         Frame Ground 1

>  Note: The mac does not provided DTR, but by 'shorting' pin 20 with
>pin 6 (see diagram) you can provide the necessary signal on this wire
>for the modem if it really insists (basically the modem acknowledges
>its own request for a DTR). 

While this will work, is necessary for modems that require DTR, I
think that most, if not all, modern high-speed modems can be
configured to ignore DTR. Thus, I prefer the setup where Mac pin 1
(HSKo) is connected both to modem pin 4 (RTS) and modem pin 20 (DTR).
That way, when using high-speed applications, the modem can ignore
DTR, but things like Appletalk Remote Access can still use it to hang
up (since ARA's script sends an AT&F to restore factory default
configurations, and does not use modem compression or speed
buffering, i.e. does not use hardware flow control).

Another nice improvement to make is to connect Mac pin 7 (general
purpose input) to modem pin 8 (carrier detect). This way, the CD line
can still be read by software that knows to look there (i.e. several
modern BBSes, some of which actually require this connection).

This leads to the following configuration:

    Mac Pin        Mac Function        DB 25 Pins       RS 232 Function

      1            Output Hand-            4            Request To Send
                      shake               20            Data Terminal Ready

      2            Input Handshake         5            Clear To Send

      3            TxD -                   2            Transmit Data

      4, 8         Ground                  1            Shield
                                           7            Ground

      5            RxD -                   3            Receive Data

      6            TxD -                   NC

      7            Misc. (Not on           8            Carrier Detect
                           Plus)

This turns out to be the "official" pinout that can be found hidden in
the current release of the Apple Modem Tool (it's a PICT you can
examine with ResEdit, or you can turn on Balloon Help while in the
configuration dialog, and move the mouse around slowly about half a
centimiter under the separator line below the "Cancel" button, until
you find the (small, hidden) region which pops up the picture. There's
another easter egg just above it.

This is also the wiring that is used by the excellent cable I bought
from Maya Computer. They're a great company and can  be reached in
many ways:

In the U.S. they have a toll-free number, 1-800-541-2318.

Otherwise, their fax number is +1 802 496 8110, voice is +1 802 496 6997

They can also be reached on Compuserve, as 75226,3632 (from the
Internet, that's "75226,3632"@compuserve.com.)

Their physical mail address is

Maya Computer Company
Bridge Street Marketplace
Box 680, Waitsfield
Vermont 05673
U.S.A.
-- 
Jim Elliott-------------------------------elliott@veronica.cs.wisc.edu
 "There is perhaps no phenomenon which contains so much destructive
  feeling as moral indignation, which permits envy or hate to be acted
  out under the guise of virtue."  -- Erich Fromm
Newsgroups: comp.sys.mac.comm