[time-nuts] FASTRAX GPS

Bob Camp kb8tq at n1k.org
Wed Jul 2 14:11:03 EDT 2014


There are an enormous number of variables in real world RS-232 (as opposed to spec compliant RS-232). 

Are you driving with >= +/- 5V or maybe with 3V?

Do you have 1 to 2 K ohm current limiting resistors on the drivers (popular in the 1970’s) ?

How big is the dead band / hysteresis on your receiver?

Is that 10 pf / foot cable or 25 pf / foot (or something in-between) in your junk box ? 

How many stop bits are you using (can it re-sync between characters?) ? 

How fast is the sample clock on your RX UART (16X or is it faster?)? 

How close are the clocks on both ends (how much distortion at zero length?)? 

Is the cable impedance controlled (like cat-5) or just “stuff” (like most commercial serial cables)?

How much of a ground offset do you have between ends / how much common mode noise?

Back a *long* time ago, we used to make dual clock XO’s that had the RX and TX speeds slightly offset to improve performance. There are lots of tricks that have been tried over the years…

All that said, I’d be surprised if you could not run 9600 baud over a 30 foot long cable, no matter how rotten it was. 


On Jul 2, 2014, at 1:45 PM, Hal Murray <hmurray at megapathdsl.net> wrote:

> albertson.chris at gmail.com said:
>> I tried using a long serial cable.  Just because I had some 100 feet of
>> cat-5 wire already installed.  It did not work reliably I was using a MAX232
>> chip as a driver.
> Were you using it as 8 separate wires or 4 pairs?
> I'd expect RS-232 to work over 100 ft of Cat-5 if you used half of each pair 
> as a ground.  (That's at 9600 baud.)
> -- 
> These are my opinions.  I hate spam.
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