23 Jan
2014
23 Jan
'14
7:23 p.m.
On 01/22/2014 07:58 PM, Eric Smith wrote:
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 went back to the 8274 datasheet and it seems that the designers
considered this to be a feature. In theory, one could simply start
the "hunt" for the next start bit as soon as the last data bit frame had
passed--I have no doubt that many chips actually work exactly this
way--that the "stop bit" is a simply an indeterminate marking period
until a space signals the start of the "start bit".
But Intel seems to say in its datasheet that a space during the period
of a stop bit is a framing error--and goes on to say that a half-bit
period is added in case of a framing error. Doubtless Intel considered
this a feature.
What I don't know is if the period of the mandatory space bit on receive
is also governed by the "number of stop bits" setting in the mode setup
word--or if, as in many UART chips, this controls only the length of the
stop bit on transmit.
Or, as some Intel applications engineer might have said, "That's not a
bug, that's a feature."
--Chuck