[ntp:questions] problem with ntpd refclock and pps via parallel port

David Lord snews at lordynet.org
Thu Feb 4 01:19:10 UTC 2010


David Lord wrote:
> Hal Murray wrote:
>>> Some rs232 receivers I've checked datasheets for are not ttl
>>> compatible. Different versions have different thresholds, some
>>> versions can have threshold changed by external resistor.
>>
>> If anybody finds a data sheet for a modern chip that
>> doesn't work right with TTL signals, please let me know
>> about it.
>>
> 
> I've only mc1489 chips to play with here, on=1.25, off=1.0,
> and mc1489A, on=1.95, off=0.8. Both those would be ok with
> my bc337 pulldown ttl output. I'm pretty sure my old printer
> required on=+3, off=-3. As for various pcs here they probably

Of course it's input current that's required which means a low
enough pullup resistor value or current drive.

David




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