Electronic Systems TRS-80 Serial I/O Board?

Chuck Guzis cclist at sydex.com
Tue Jun 13 18:53:34 CDT 2017


On 06/13/2017 02:26 PM, Eric Smith wrote:
> On Tue, Jun 13, 2017 at 12:39 AM, Chuck Guzis via cctalk

> A person might think so, but as DEC found out with the PDP-11/05
> console serial port, it's really not. The percentage tolerance of
> async serial is not any higher at low bit rates than at higher bit
> rates, and the percentage tolerance of RC oscillators isn't any
> better at low frequencies than at higher frequencies.  Being off by
> only a few percent is enough to be a problem, because the other end
> might be off by a few percent also. 1% resistors are dirt cheap now,
> but they weren't in the 1970s.  It's become only slightly easier
> since then to get capacitors with 1% tolerance and low tempco.

That doesn't jibe with my own experience.   When I got my TV Typewriter
going, I was too cheap to buy the extra components for the serial board
to enable multiple bitrates.   Instead, I cobbled up a 555 operating in
astable mode with a simple trimmer pot for adjustment (not even the
10-turn units used in the example).  It was plenty stable.

The TR1602 UART, like its cousin, the AY-3-1013 used in the TVT,
tolerates a pretty wide range of bit rate distortion.  The app note
gives a figure of something like 49%.  And, since it's async, the game
starts all over at the next character.

Maybe DEC wasn't using the same sort of UART; I don't know.

--Chuck




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