On Tue, Jun 13, 2017 at 12:39 AM, Chuck Guzis via cctalk < [email protected]> wrote:
> The trimpot on the board says to me that the clock is most likely a > simple RC affair. That does seem likely. > 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 tempco. 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.
