NEC uPD7201 misfeature, and V.14 (was Re: NEC uPD7201C Datasheet, Pinout?)

On Jan 22, 2014 6:58 PM, "Chuck Guzis" <cclist at sydex.com> wrote: almost 202 the similar, but obviously not identical. However, if you've programmed one, you'd be right at home with the other. always used 8251A or 2651/2661 chips, with a preference for that latter. The described problem occurs when using async mode, not sync, but when using a modem that does synchronous modulation, which is all modems that use modulations fancier than FSK, i.e. voice-band POTS modems at 2400 bps and up, and full-duplex 1200 bps, and when NOT using MNP or V.42 error control implemented in the modem. This means that it was a common occurrence when using the uPD7201/8274 with early 212, V.22, and V.22bis modems. It mostly was not seen with V.32 and later modems because those almost always use V.22 error control. I'm not sure what other common microcomputer products used the uPD7201/8274, but this problem was very well known in the mid-1980s among AT&T 7300 and 3B1 users. It was especially aggravating if you had spent the money for a modem with MNP and/or V.22, but the modem bank you called into did not support it. The 8251A, 2651, 2661, 2681, 6551, 6850, 68681, Z80-SIO, Z8030/8530 SCC, and almost every other UART and USART in the known universe did not have this problem. It was a poor design choice (ay least in hindsight) by the uPD7201 designers, in not allowing their chip to accept a slightly short stop bit. I think everyone else accepted not due to planning for V.14, but simply because they copied the sampling method used by the earliest UARTs, which just happened by chance not to be affected by slightly short stop bits. Of course, if UARTs that couldn't tolerate short stop bits had been commonplace, the CCITT would probably not have promulgated stop bit shaving in V.14, and would have had to come up with a different method of rate matching.