From: cdl@triton.ucsd.edu (Carl Lowenstein)
Newsgroups: comp.sys.next.marketplace,comp.sys.next.hardware
Subject: Re: Modem cable
Date: 30 May 1995 11:05:23 -0700
Organization: Marine Physical Lab, UC San Diego

In article <3q0et2$r7k@service1.uky.edu> rjhorn@news-host.ms.uky.edu (R.R. Hornback) writes:
>Shaun Patrick Foy (sfoy@bcu.ubc.ca) wrote:
>: Hi,
>
>: I have a NeXTstation computer and a USRobotics modem. Apparently, the
>: modem cable that USR distributes with their Mac&Fax modems does NOT
>: support hardware flow control :( I need a next modem cable. Anyone
>: have one? Anyone know where to get one?? (quickly??)
>
>Someone correct me if I am wrong, but I believe you can simply 
>use a standard Mac cable that does hardware flow control.  You can
>pick these up a a store like Best Buy for 5 or 6 bucks...

OK, you're wrong.  The DIN-8 connector on a NeXT serial port looks
just like the DIN-8 connector on a Mac serial port, but the signals
have not had the same pinout since the long-ago days of the 68030 NeXT.

Here's a table showing how to make a modem cable for Mac (irrelevant)
and NeXT.  The paragraph of text shows the convolutions necessary to
make hardware handshaking work on a Mac.

A hardware handshake cable allows the Macintosh to assert both the DTR
and RTS handshaking signals with its HSKo (handshake-out) control
line.  The HSKi (handshake-in) control line allows the computer to
monitor the Clear to Send (CTS) signal from the modem.  Since DTR is
tied to RTS, you must have the modem set to ignore any state change of
DTR, otherwise an RTS flow control change would cause the modem to hang
up the telephone line.


Mac Pin and Function     NeXT Pin and Function  DB25 (RS232) Pin and Function
-----------------------  =====================  -----------------------------
Pin 1 (Handshake Out)    Pin 6 (RTS)            Pin 4 (Ready to Send)
Pin 1 (Handshake Out)    Pin 1 (DTR)            Pin 20 (Data Terminal Ready)
                         Pin 2 (DCD)            Pin 8 (Carrier Detect)
Pin 2 (Handshake In)     Pin 8 (CTS)            Pin 5 (Clear to Send)
Pin 3 (Transmit Data)    Pin 3 (TXD)            Pin 2 (Transmit Data)
Pin 4 (Ground)           Pin 4 (GND)            Pin 7 (Ground)
Pin 5 (Receive Data)     Pin 5 (RXD)            Pin 3 (Receive Data)
Pin 6 (TXD+) No connection
Pin 7 (NC)
Pin 8 (RXD+) GND


1-gris--2
2-jaune-1
3-rose--
4-brun--
5-blanc-
6-bleu--
7-vert--
8-rouge-
Mac Pin and Function        NeXT Pin and Function
-----------------------     =====================
Pin 1 (Handshake Out)       Pin 6 (RTS)
                        NC  Pin 2 (DCD)
Pin 2 (Handshake In)        Pin 8 (CTS) 
Pin 3 (Transmit Data)       Pin 3 (TXD)
Pin 4 (Ground)              Pin 4 (GND)
Pin 5 (Receive Data)        Pin 5 (RXD)
Pin 6 (TXD+)            NC
Pin 7 (NC)
Pin 8 (RXD+)           GND


Mac Pin and Function     NeXT Pin and Function  DB25 (RS232) Pin and Function
-----------------------  =====================  -----------------------------
Pin 1 (Handshake Out)    Pin 1 (DTR)            Pin 20 (Data Terminal Ready)
                         Pin 2 (DCD)            Pin 8 (Carrier Detect)
Pin 3 (Transmit Data)    Pin 3 (TXD)            Pin 2 (Transmit Data)
Pin 4 (Ground)           Pin 4 (GND)            Pin 7 (Ground)
Pin 5 (Receive Data)     Pin 5 (RXD)            Pin 3 (Receive Data)
Pin 1 (Handshake Out)    Pin 6 (RTS)            Pin 4 (Ready to Send)
                         Pin 7 (NC)
Pin 2 (Handshake In)     Pin 8 (CTS)            Pin 5 (Clear to Send)
Pin 8 (RXD+) GND
Pin 6 (TXD+) No connection


Notice several things:

The Mac uses pin 8 of the DIN connector for RXD+, as part of its balanced
RS422 data transmission.  To connect a Mac to RS232, this pin must be
grounded.  The NeXT uses pin 8 for CTS.

The Mac uses pin 6 of the DIN connector for TXD+, as part of its balanced
RS422 data transmission.  The NeXT uses pin 6 for RTS.

The Mac has only one Handshake Out line (DIN pin 1) and must use it for
both RTS and DTR (4 & 20) on the DB25 connector.

The Mac Handshake In line (DIN pin 2) is functionally equivalent to the
NeXT CTS line (DIN pin 8).

The Mac does not have anything equivalent to a DCD input.

The only time you can get a Mac serial cable to work on a NeXT is if
you use only DB25 pins 2, 3, and 7, connected to DIN pins 3, 5, and 4.
That is, no hardware handshake.

Everybody repeat after me:  "A Mac serial port is not the same as a 
NeXT serial port, even though they use the same type of connector".

For confirmation of the way to connect a NeXT to a modem, just ask your
NeXT for "% man zs".

    carl
-- 
        carl lowenstein         marine physical lab     u.c. san diego
        {decvax|ucbvax} !ucsd!mpl!cdl                 cdl@mpl.ucsd.edu
                                                  clowenstein@ucsd.edu
