Serial floppy drives

From: Eric Smith <eric_at_brouhaha.com>
Date: Wed Sep 4 20:02:00 2002

> I always thought that was called RS-422.
>
> The Macintosh standard serial port was a RS-422 serial port. When I
> went to look up what that meant, the only difference I could find was
> the voltage levels.
>
> RS-422: 0v to 5v
> RS-232: -5v to 5v

There's a lot more difference than that, and the voltage levels quoted
are wrong, or at the very least, misleading. I don't have a copy of the
relevant standards handy, so I might have some details wrong, but here
goes...

RS-232, which is now called EIA-232, is a single-ended signalling scheme.
At the transmitter, the voltages are nominally around +12V (mark) and
-12V (space), but are actually allowed to range from +25V to -25V. Aside
from voltage output, there are various other requirements on the
transmitter including short-circuit protection.

At the receiver end, voltages above +3V are considered a mark, and below
-3V are considered a space. A signal between +3V and -3V does not meet
the EIA-232 specs, so the behavior is undefined, but most receivers will
consider a signal near 0V to be a space.

> The documentation I found went on to say that RS-422 was backaward
> compatiable with RS-232.

The documentation is wrong. EIA-422 is a differential signalling standard.
Rather than having a data signal referenced to ground, you have two
signals referenced to each other, e.g., RxP and RxN for receive, and
TxP and TxN for transmit. EIA-422 cannot interoperate with EIA-232
without a converter of some kind.

However, the Macintosh ports also support EIA-423, which is a single-ended
signalling method using levels approximately equivalent to those of
EIA-422. In many cases EIA-232 equipment and EIA-423 equipment can
interoperate (as is usually the case with Macintosh serial ports and
EIA-232 devices), but technically an EIA-423 device does not in fact meet
EIA-232 specifications.

Another difference between EIA-232 and EIA-422 is that EIA-232 specifies
a connector and the assignment of functions to connector pins, while
EIA-422 and EIA-423 specify electrical characteristics only.

Note that a device that uses an EIA-232 electrical interface but not
the DB-25 connector is not actually compliant with the EIA-232
specification. For instance, devices with DE-9, modular, or MMJ
connectors.
Received on Wed Sep 04 2002 - 20:02:00 BST