[time-nuts] Rapco 1804M - serial problems?

Hal Murray hmurray at megapathdsl.net
Mon Oct 17 15:43:50 EDT 2011

> I've just got a Rapco 1804M and while it works, within the limits of my
> antenna placement, the RS-232 serial out doesn't read very well, and seems
> to have incorrect characters.  The voltage levels look OK, and the  terminal
> emulation program is set to 4800 baud, 8-bit, no parity, one stop  bit, no
> flow control.  I've tried other combinations.  As near as I can  measure,
> the baud rate is 4800.

Do you have a scope?

I'd check the voltage levels, the bit timing, and the rise/fall times.  You 
can also check the timing on the receiving system by getting it to send 

Sometimes the UART uses a handy clock that doesn't generate exact 
frequencies.  Usually that's only a problem at faster baud rates.

If the driver is weak, the rise time might be a problem.  (But 4800 is very 

If that all looks good, I'd expect there is some sort of noise.  How long is 
the run?  How close are the grounds?  Try disconnecting the DB-9 at the 
receiver and putting the scope on the ground pin.

These are my opinions, not necessarily my employer's.  I hate spam.

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