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
