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From: prindle@NADC.ARPA
Newsgroups: comp.sys.cbm
Subject: 1200+
Message-ID: <2955@rutgers.rutgers.edu>
Date: Mon, 3-Aug-87 22:21:44 EDT
Article-I.D.: rutgers.2955
Posted: Mon Aug  3 22:21:44 1987
Date-Received: Tue, 4-Aug-87 06:36:54 EDT
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From: prindle@nadc.arpa (Frank Prindle)

First, thanks to Fred Bowen for his info and interest.  Fred, your now famous
lists of 1571 and C128 bugs even got mention in Jim Strasma's column in RUN
mag this month.  Too bad magazines have to be 6 months behind the rest of the
world.

Next, a bit of Commodore lore concerning the C64.  It has been told that the
reason the C64 does not have a UART is not related to cost; rather, when the C64
(or was it the VIC-20 - one or the other) had to be on the market by some drop-
dead date or other, Commodore's semiconductor division (MOS Technology) did not
have the specific UART chip into sufficient production to support the computers.
A quick and dirty *software* UART was thrown together, which is what we have
today (it survives from VIC to 64 to 128 for compatibility).  Quick and dirty
means that not much thought was put into the bit timing routines (which work
off of one of the CIA interval timers); interrupt service times for the send
and receive processes were not properly estimated or compensated for (especia-
lly when bits are being sent and received at the same time).  It all sort of
hung together by a shoestring for bit rates of 600 or less.  More correct
software UARTs (such as written by Steve Punter, for example) can handle 1200
bps full duplex with high accuracy.  At 2400 up, it becomes nasty territory
because the VIC-II chip burst DMAs from memory make it difficult to time bits
spaced less than 500 microseconds apart.  I hear the C128 in 2Mhz mode with
the VIC-II chip disabled can do a nice job at 2400 BPS, though I haven't seen
it for myself.

Anyway, you can believe either story about the UART I suppose (cost or
availability); the truth may never be known.  The only solution is to live
with what we have or buy one of those add-on cartridges which have a live
UART (and maybe modify software to work with it!).

Sincerely,
Frank Prindle
Prindle@NADC.arpa