New acquisition...

Eric Smith eric at brouhaha.com
Wed Aug 11 02:14:31 CDT 2010


Tony Duell wrote:
> Nice machine. The QX10 is one of my favourite desktop CP/M machines. The
> video system is based round a 7220, the serial port (IIRC) uses a 7201.
>    
Using a uPD7201 for the serial port is a strike against it.  High speed 
modems that transport asynchronous data over a synchronous modulation, 
such as V.22, V.22bis, V.32, etc., have to use V.14 procedures to handle 
cases where the transmitting device is overspeed relative to the 
modulation, or the transmitting modem is overspeed relative to the 
receiving side.  In the former case, up to one out of ten stop bits get 
dropped over the modulation, and reinserted on the receiving side.  When 
that happens, or in the case that the transmitting system's modulation 
is faster than the receiving side (due to crystal tolerances, etc.), the 
receiving side is allowed to shave off a portion of each stop bit on the 
serial interface.

That works fine with 99.9% of UARTs out there, which only require 9/16 
of a stop bit for a valid receive character, and to start watching for 
the next start bit.  The uPD7201 requires a full stop bit, so is not 
compliant with the V.14 standard, and will garble data and report 
framing errors when V.14 stop bit shortening occurs  This was a huge 
problem for people using V.22bis or V.32 modems with the AT&T 7300 or 
3B1 Unix PC, as they used the 7201.

It's less of a problem since the adoption of V.42 error control, because 
when using V.42 there is normally hardware flow control at both ends, 
and V.42 can apply "back pressure" across the modem link to slow down a 
transmitting system as needed.

Eric




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