Newsgroups: comp.sys.mac.hardware
Subject: Re: Modem cable myths
From: scasterg@waltham.columbus.oh.us
Date: Mon, 11 Jan 93 22:54:13 -0500
X-Mailer: rnMac Buggy, I mean Beta, Test Version

jh> scasterg@waltham.columbus.oh.us writes regarding hardware
jh> "handshaking": 
jh>   
jh>   > This is important to know if you are using a 
jh>   > Mac Plus, Classic and a couple other models 
jh>   > (LC?) which will simply not support hardware 
jh>   > handshaking due to braindead design. 
jh> 
jh> If we are trying to clear up myths, then let's clear up the two
jh> created by this sentence: 

Thanks. sorry for screwing that up.

jh>  What the Plus lacks is the serial port's pin-7 connection, used
jh> on other Macs for connecting to the modem's DataCarrierDetect
jh> output. This is nice to have if you are running a host system,
jh> since it provides a way to determine (via hardware) if a caller
jh> has hung up, but it is totally unnecessary for implementing
jh> flow-control. The GPI connection is missing from the Mac Plus,
jh> Mac Classic, Mac Classic II, Mac LC, and Mac LC II. 
jh> 
jh>   > Finally, a reliable source for hardware 
jh>   > handshaking cables: Mac Dataflow (410) 
jh>   > 569-7636 Just ask for "cable ID 3 from the 
jh>   > Hermes docs." 
jh> 
jh> I am not familiar with what wiring results from following "cable
jh> ID 3 from the Hermes doc". ( But it sure sounds like the kind of
jh> stuff myths are made of!)

No myth on this one. I can guarantee it. I've used these cables for hardware 
handshaking on th efollowing brands of modems:

US Robotics
Telebit
Intel
Quickcomm
Zoom
Atrie

The cables work flawlessly on all of them. Here are the pinouts for the 
tech-minded:

CABLE ID 3:
Macintosh MINI DIN-8 Male	  --------> 	RS-232 DB-25 Male
					1--------> 20, and 4
					2--------> 5
					3--------> 2
					8,4------> 7
					5--------> 3
					7--------> 8

This cable does, as you can see, support wiring from modem pin 8 to mac pin 7,  
but it also implements hardware handshaking by connecting RTS(4) and DTR(20) on 
the modem to DTR(1) on the Mac (I think I got that right:-)

Hardware handshaking cables may be widely available if you know where to look 
for them, but I've seen more than one user here say that he went down to his 
local Mac retailer and asked for a hardware handshaking cable only to find out 
that they did not know what he was talking about. This cable does provide that 
without having to hunt around.


 Before pointing people in that
jh> direction I think one ought to specify what connections one gets
jh> from this arrangement. Cables wired for hardware flow control are
jh> quite abundant these days. What is harder to find is cables that
jh> contain wiring from the modem connector's pin 8 (DCD) to the
jh> mac's pin 7 (GPI). But not many people need that connection in
jh> their cable. (Of course, if you do have such a cable, you need
jh> software that will pay attention to the status of the DCD; 

If your software doesn't use it, it will just ignore it. At least, that has been 
my experience.

to my
jh> knowledge, there isn't any Mac comm software that does. One
jh> exception to this is the Server software of FirstClass. It has an
jh> option for using DCD via mac pin 7 to monitor the status of the
jh> modem carrier.)