Electronic Systems TRS-80 Serial I/O Board?

Chuck Guzis cclist at sydex.com
Tue Jun 13 19:59:03 CDT 2017


On 06/13/2017 05:29 PM, Eric Smith wrote:

> It's mathematically impossible for a normal UART [*] to handle 49% 
> timing error. The cumulative timebase error by the end of a
> character can't be more than one bit time, or the wrong bit will get
> sampled, resulting in incorrect data, or, (if that happens on the
> stop bit) a possible framing error. For 8N1 [**], there are 10 bits
> in total (including start and stop), so that's an absolute maximum
> timing error of 10%, but for various reasons even 10% speed variation
> won't actually work in practice. If they meant 4.9%, that is
> believable, but even that won't work if the other side is more than
> slightly off-speed in the other direction. Normal spec is a maximum
> timing variation of within +/-2% at each end [***], so that things
> still work properly if one side is at +2% and the other is at -2%,

Well, I didn't say "timing error", I did say "timing distortion", which
is not quite the same thing.  My reference was the "TR1602/TR1863/TR1865
MOS/LSI Application Notes Asynchronous Receiver Transmitter", which can
be found in the WD 1984 Data Communications Handbook (I think there's a
copy online). Page 126-127.  "Thus, signals with up to 46.875%
distortion could be received."  Perhaps this is in error; I'll let you
decide.

I can say, however, that once I set the trimmer on the 555, I didn't
touch it for a couple of years (eventually replacing the TVT with a real
terminal).  Obviously, the developer of the subject board didn't have
much of a problem either; or else he wouldn't be able to sell the thing.

--Chuck



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