Subject: Re: IIGS Serial ports
Newsgroups: comp.sys.apple2
From: dempson@actrix.gen.nz (David Empson)
Date: Tue, 30 Nov 1999 03:05:28 +1300
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Supertimer <supertimer@aol.com> wrote:

> Frank Carney <frankc@cyberhighway.net> wrote:
> 
> >Then how can it drive an external modem?
> 
> RS-422 and RS-232 differ in connectors and the fact that
> under RS-422, the DTE vs. DCE can be reversed inside
> the cable.

Noooo....

RS-422 does not have a standard connector (or if it does, it isn't the
Mini-Din-8 used by Apple).  It is an electrical standard defining a
balanced transmission system (using "true" and "inverted" signal lines)
with voltages in the region of +/-5V.  (If there is a standard
connector, this connector will also define the signals.  I've only seen
it in connection with the ITU X.21 standard, which is used with some
digital wide area networks.)

RS-232 defines both the electical characteristics (signals ranging
between +3V and +15V or -3V and -15V) as well as the DB-25 connector and
pin functions.

The main electrical difference between RS-422 and RS-232 is the fact
that 422 has a pair of wires for each signal (tx data, rx data, etc.),
the voltages are different, and 422 can go a lot further and faster than
232.  (422 is limited to about 1000 metres at speeds up to the region of
1 Mbps, while 232 is limited to about 15 metres at 9600 bps.)

There is also a variant of RS-422 called RS-423, which is a single-ended
version running at the same voltages.

The IIgs and Mac serial ports are actually combined RS-422 and RS-423
ports: the data lines are RS-422 (a pair for each signal), and the flow
control lines are RS-423.  In addition, the data lines can be treated as
RS-423 by ignoring the positive transmit line and grounding the positive
receive line.

RS-423 and RS-232 are reasonably compatible, as long as there isn't much
voltage drop from the RS-423 transmitter to the RS-232 receiver, and the
RS-423 receiver is able to deal with the wider voltage range used by
RS-232.

> IIGS and Mac modem cables have an RS-422 connector
> on one end and an RS-232 connector on the other.

The only thing that is special about the IIgs and Mac serial ports is
that Apple very sensibly decided to use the same pinout for all devices
(at least computers and printers, and possibly also modems) which use
the Mini-Din-8 connector.  This means that all cables for connecting
Mini-Din-8 devices are "cross-over" (null modem) cables.

There is one exception to this: extender cables are "straight through",
and switch boxes generally require the use of an extender cable on one
side.  (I modified mine so that it requires a cross-over cable on both
sides.)

When dealing with RS-232 devices (with a DB-25 or DE-9 connector), the
Mac has to take the signal polarity into account, so you end up with two
cables: one has the correct pinout to plug into a DTE (e.g. a printer)
while the other has the correct pinout to plug into a DCE (e.g. a
modem).

High-speed modems with hardware handshaking add further confusion, since
they use the flow control signals in ways which are not defined by the
RS-232 standard, resulting in "unusual" wiring of the cable.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P O Box 27-103, Wellington, New Zealand