>>- Do they talk to a PC or other device with a crystal controlled
oscillator or to other units the same?
They talk to a PC at 4800bps and some of them talk to a 12F629 running
on a internal RC too.
>>


A UART stays in synchronisation with up to half a bit slip in 10 bits or so
or about +/- 5% error.
Add an RC clock at the other end and it can be +/- 2.5% if the errors are
not correlated. (ie if temperature rises cause clock speed to change the
same way at both ends the effective error will be decreased).

That suggests a *real world* RC clock accuracy across all variable of under
+/-2%.
Maybe we can trust the spec sheet sometimes :-)



        Russell McMahon

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