Apple II hardware design

From: Frank McConnell <>
Date: Sun Jun 22 23:34:50 1997

"A.R. Duell" <> writes:
> I've never liked bit-banged serial ports either (except on
> microcontrollers). They always seem to have problems with full-duplex
> operation. Yes, Apple sold a bit-banged serial port for a time - I have
> one with the manual (which, amazingly contains instructions on linking it
> to an ASR33), presumably to save a UART chip. They then sold one that
> worked properly (in full-duplex mode, etc) under the name 'super serial
> card'. It used (IIRC) a 6850 chip (or was it a 6551?)

Hmm? What was that bit-banged serial port?

If you look in the Apple ][ red book, there is a little circuit
in there that plugs into the game I/O connector and drives a
20mA current loop. Alongside there is a short assembly program
to drive it. Now there is a bit-banger.

There was an unspectacular serial card for the ][. I don't recall it
being a bit banger, just that the combination of it and the printer I
was using at the time (an IDS BrighterWriter) wasn't smart enough to
manage any sort of common flow control, so that I had to run it at 300
baud. I thought it had some sort of UART-like thing, but maybe my
brain is going again.

Hmm, I think it was called the Asynchronous Serial Interface or
something like that. There was also a Synchronous Serial Interface
that (I recently found out) was the Silentype printer interface.

I don't remember the Mountain Hardware CPS card that well, and I feel
very good about that based on what I do remember. Now there was a

The Apple Super Serial Card was designed around a 6551.

There were a couple of other cards designed around the 6850.
The Hayes Micromodem ][ was one of these.


Strange as it seems today (now that I have done some programming
around PC-contemptible serial ports), the Apple ][ serial cards and
software generally worked by software-polling-hardware. The only
serial card I can remember supporting interrupts was the Super Serial
Card, and I can't say that I ever saw it used that way. Certainly
none of the "standard" software required it; interrupts just weren't
generally done on Apple ][s.

-Frank McConnell
Received on Sun Jun 22 1997 - 23:34:50 BST

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