Electronic Systems TRS-80 Serial I/O Board?
bhilpert at shaw.ca
Tue Jun 13 17:02:18 CDT 2017
On 2017-Jun-13, at 2:26 PM, Eric Smith via cctalk wrote:
> On Tue, Jun 13, 2017 at 12:39 AM, Chuck Guzis via cctalk <
> cctalk at classiccmp.org> wrote:
>> The trimpot on the board says to me that the clock is most likely a
>> simple RC affair.
> That does seem likely.
There's a 7493 (4-bit counter) on the board as well, which looks to have connections to the dip switches beside it,
in all likelihood the baud rate divider and rate selection, and so the clock would be running at higher frequency.
Simple board to RE if one had a need for it.
>> For low bitrates, that's perfectly adequate.
> 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
> DEC went through multiple board revisions with changes to the RC oscillator
> in attempt to make it sufficiently reliable. I've heard that they finally
> up and putting a crystal oscillator on the board, but all of mine have the
> RC, and they've given me some grief over the years. I replaced one with a
> crystal oscillator.
HP also used RC timing for serial on early versions of the HP2116 serial interfaces, and later went to crystal as the speeds went higher,
but then HP also had a habit of using high-quality componentry (e.g. 1% resistors when one could wonder why).
I expect the RC stuff was largely done in expectation of use with teletypes/33s.
Better was known obviously (crystals) but RC was not uncommon in the era.
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