Subject: Re: SuperSerialCard Question!
From: dempson@actrix.gen.nz (David Empson)
Date: Tue, Oct 27, 1998 3Ç32(H
Message-id: <1dhkziu.taw6nc1wpquicN@dempson.actrix.gen.nz>

Supertimer <supertimer@aol.com> wrote:

> The Super Serial Card came with a faulty 6551.  You can
> get it replaced from the people who make the TurboASB.

Ah, I know what the problem is.

The 6551 is not "faulty".  The problem is that the 6551 has a peculiar
design feature: if the incoming flow control signal (CTS) turns off, the
transmitter is immediately disabled, causing a character being
transmitted to be cut off, producing a garbage character at the other
end.

This applies to the original Rockwell R6551, and possibly to some second
sources.  The Rockwell R65C51 (CMOS version) and other second sources of
the 6551 will wait until the current character is completely sent before
stopping transmit in response to CTS going away.

The outgoing flow control signals cause similar problems (on all
versions): dropping RTS will immediately cut off the transmitter, but
this can be managed by waiting for the current character to be
completely sent before RTS is dropped.

The replacement 6551 provided with the TurboASB will behave like the
R65C51, so hardware handshaking doesn't cause data corruption.

> >> Thank God the IIGS got something better.  In fact, the IIGS'
> >> serial ports are identical to Macintosh ones.  ;-)
> >
> >Not quite.  The IIgs is missing one of the external clocking options
> >(but still has the clock-via-CTS which is used with the Apple MIDI
> >interface), and is nowhere near as capable as later Mac ports, which
> >support 115200 bps and DMA.
> 
> What I meant was identical to Macintoshes until recently.
> Identical to the "56k" Mac ports, the ones that were used
> in the SE, Mac II series, LC series, Centris series, etc.

Except for the clock-via-GPI option, which is available on the SE and
later machines, if I remember right.  (The Mac Plus doesn't support GPI,
which is usually used as the carrier detect input on the IIgs and on
most later Macs.)

> Of course it is not the same as an iMac serial port!

What iMac serial port?  :-)

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