Different twist on a mac serial Cable

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Wesley Konrad - 26 Jun 2007 02:17 GMT

I know there is all kinds of information on making your own mac din8
serial cable to a DB9 pc serial cable but my question is a bit
backwards.

In all the examples I have seen online, the cable is made from the
perspective of having the Din8 come out of the mac to the DB 9 on the
device. I have the opposite problem.

I have a US Robotics Sportster Voice Modem. This was a model
(#00114100) that was made specifically for Macs, which means that it
doesn't have DB25 like most external modems do, it has a din8.

I want to get it working on a bondi imac which doesn't have a serial
port. I do however have a Keyspan USA-19QW USB to Serial adapter.
The adapter is a DB9 though. So, instead of going through the expense
of purchasing a new USB adapter I want to just make a cable.

Is there any reason this shouldn't work? Like I said, all the stuff
I've seen online talks about going the other way.

I tried contacting US Robotics but they seem too clueless to even give
me a serial pin out on the modem so I'll assume it is the same as a
standard mac serial port.

Any help would be appreciated.

Wesley

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Eric Bear Albrecht - 26 Jun 2007 17:29 GMT

> I know there is all kinds of information on making your own mac din8
> serial cable to a DB9 pc serial cable but my question is a bit
[quoted text clipped - 23 lines]
>
> Wesley

That is a very strange modem. It's the gray one, right?
On a Mac with a serial port you need to use a non-Mac cable, i.e.
straight through.

The deal is that when Apple looked at this mess they call RS-232 it was
obvious that the designers of the protocol were thinking way inside the
box ... the pin assignments were based on the assumption that either
you're a terminal or you're a modem. (Computers were huge roomfuls of
stuff that you were not allowed to even get close to.)

So, given that lots of confusion had come along when the world changed,
some rethinking was in order. Apple's way of looking at it was much
more general and sensible -- either you're a cable or you're a box. So
the signal goes into the cable on one pin and comes out of it on the
other. This always works, with no adapters.

Of course the PC world paid no attention, and when they went to use
these cute little mini-DIN connectors they did it the dain-bramaged way
they always had. Which is why when you get a switch to expand one
serial port to two or more, internally it's PC-style wiring. So you use
normal Mac-style cables with the reversal on one side of the switch and
a straight-through cable on the other. Normally that would be Mac
cables on the output side and a single straight-through between the Mac
and the switch, but either way works. Normal is defined by what the
usual Mac person would have on hand, like not a bunch of straight-
through cables.

This brings up another method you could use: if you have an old serial
switch laying around, you could hook up that modem with two normal Mac
serial cables.

For building your own cable, you can work out the details now that you
know that the modem is bass-ackwards.

HTH

Signature

Bear Albrecht
San Antonio NM

Reply to this Message

You - 26 Jun 2007 23:48 GMT

> I know there is all kinds of information on making your own mac din8
> serial cable to a DB9 pc serial cable but my question is a bit
[quoted text clipped - 23 lines]
>
> Wesley

First off, your modem is a RS422 serial connection, instead of a RS232
connection which your looking for.
Second off, RS422 can be made to function as RS232 in short cable runs.
Third off, the exact wiring your looking for is available in the
<Macintosh Family Hardware Refernce> Book First Edition....
Fourth off, your lucky that some folks still have a copy of that book
around....
Fifth off, .......Oh Hell.....I can't remeber what should be fifth....

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Mac Forum / General / Hardware / June 2007

Different twist on a mac serial Cable