HP 21MX/F and OS choices (was Lookee what I just got!)

Brent Hilpert hilpert at cs.ubc.ca
Wed Jan 10 22:51:44 CST 2007

Bob Shannon wrote:
> But 12531 boards are slow, and not the most reliable.  They often drop
> characters when jumpered for higher baud rates.

I use a 12531 for downloading into my HP2116 and experienced the dropped
character problems when I first started. I found a simple solution however:
set the transmitting machine to send two stop bits (if one's transmitter
provides the option of course).

The problem with the 12531 is that - in addition to not being double-bufferred -
it doesn't indicate receipt of a character to the software until the end of the
first stop bit. With only one stop bit, the software then has only a fraction of
a bit time to pick up the character before it must restart the 12531 to receive
the next character, so it can sync on the already-begun start bit. If you send
two stop bits then the software has the entire period of the second stop bit to
play with.

I download multi-KByte streams into the 2116 at 19200 BPS without problem.

http://www.cs.ubc.ca/~hilpert/e/HP21xx/io12531BCD.html includes some notes on
jumpering for higher baud rates and a cable schematic.

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