Re: Original Apple IIc serial inaccuracy (?)

[Mitchell Spector <mitch2gs@hotmail.com>]

schmidtd <schmidtd@my-deja.com> wrote:

>So, my $12.01 IIc arrived a couple of days ago.  It turned out to be
>an original ROM 255 model, with the 74LS161 chip instead of the
>oscillator.  So, it should have the 3% timing deficiency with its
>serial ports.  (ref: http://en.wikipedia.org/wiki/Apple_IIc#Serial_port_timing_fix)

    I actually wrote that Apple IIc article on Wikipedia (including the bit
about the serial port timing issue) but re-reading it now, it could use a
some clarity. More below...

>Strangely enough, trying out ADTPro with it on both Windows and OSX,
>this little IIc was able to transfer disks back and forth flawlessly
>all the way up to 115.2kbps.  Have computers become more tolerant of
>timing inaccuracies over time?  Or was the problem only with more
>hardware-ish devices like modems and printers?

    It only affected specific manufactures. For example the Apple 
1200 baud modem had no issues with the serial port, but certain 
third party modems (e.g. Hayes, USR, Packard Bell) rated at 1200 
baud or higher may have run into problems. It was really a hit or 
miss thing, as the timing deviation was *just* outside the specified 
operating range. Some  devices could cope, others could not.
Apple unfortunately only tested their own modems at the time.

    I don't think it affected serial printers as much as it did modems
(if at all), as they were less particular about timing. I remember some
documentation of the issue claiming it only affected port 2 (modem) 
but then the same 6551 ASIC did control both ports 1 and 2. At best
guess they were simply referring to modems as that is what normally 
would connect to port 2.

    In a nutshell I would say the serial port on your PC and Mac are
able to tolerate the slight timing devation (just as those Apple brand, 
and some certain third party modems of the past did). Remember not 
every IIc owner found themselves affected by the glitch at the time.

Mitchell Spector