James Knott wrote:
NoOp wrote:
On 05/07/2008 11:59 AM, James Knott wrote:
mike scott wrote:
On 6 May 2008 at 17:15, James Knott wrote:
Jerry Feldman wrote:
...
It was 1.5 stop bits back in 1976 when we were integrating a 1200bps
modem into our POS system. I'm not talking about Baudot code. In any
...
It was never 1.5 stop bits for ASCII devices. It was either 2 stop
bits at 110 b/s or 1 at 300 & above. It wouldn't hurt anything,
other than
I'm not sure that's correct. I'm pretty sure it was an option (1,
1.5, 2) on VAX serial ports.
The option may have been available, as it was on the 8250 UART used
in
I've a few of those, and 8251 USART's out in the garage... I know the
8251 could be configure to allow 1.5 stop bits, can't recall if the 8250
also had this option. However, 1.5 was only used for characters with 5
information bits, so it wouldn't apply to standard/modern modems anyway.
On the 8250 and later, you specified the number of data bits and stop
bits separately, so it was possible to have any combination of data bits
and stop bits. However, in all my years of experience with working with
this stuff, I have never, not once, seen 1.5 stop bits used with 8 bit
codes. It's always been either 1 or 2 stop bits, though 1.42 and 1.5
were common with 5 bit codes. I've also never seen a UART that could do
1.42 stop bits, which is found only on mechanical devices. A UART would
be configured for 1.5 stop bits, which isn't much of a difference from
1.42. So, while it is technically possible to configure for 8 data bits
and 1.5 stop bits, it's never, to my knowledge, ever been done in
production systems, as it doesn't comply with any of the standard
codes. As I mentioned in another note, this is one area, having worked
for 36 years in data communications and computers, that I have a *LOT*
of experience. In fact, when I designed & built that 8 port serial card
I mentioned earlier, I found a bug in the 8250 that National
Semiconductor didn't know about. I even got a nice letter from them for
finding it.
The fractionally longer stop bit was a carryover from the mechanical
teleprinter days. Before synchronous motors became common, shaft speeds
were controlled by a governor. No matter how carefully they were
adjusted, there was always some variation between units. So the stop bit
was lengthened to allow the slower device to stay in sync with each
character. I believe this applied to all devices up through the early
Model 28 equipment, somewhere in the mid 1950's. The extended bit was
added to the transmit side of early electronic interfaces as well, just
in case there was a mechanical device at the other end. Variations
included 1.42, 1.5 and 2.0 bits.
For more information about mechanical teletypes, here are some web sites
and an active mailing list for former TTY repairmen like myself.
<http://www.nadcomm.com/>
<http://www.rtty.com/>
<http://mailman.qth.net/mailman/listinfo/greenkeys>
Bob McConnell
N2SPP
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