[time-nuts] LeoNTP PPS output to PC DB9 input

[Hal Murray]

tom at aussec.com said:
>> Alternatively, is there a simpler way to achieve the goal?
> No, you really need a level shifting stage that
> converts the PPS to RS232 levels. 

I've been running a PPS from a TBolt directly in to a RS232 without a level 
shifter for years without any problems.

In the old old days, the 1488 and 1489 were the TTL to RS232 level shifter 
chips.  (I forget which was transmit and which was receive.)  The receive side 
had an input threshold of roughly 1.4 V, same as TTL logic.  All the followup 
replacement chips kept that property.  Too many people depend on things like 
this working.

On a lab bench, I'd expect things to work fine without a level shifter.  It 
won't work as well with long cable runs or crappy grounds.

If I was doing something like you want to do, I would do something close to 
what Brent Gordon described.  I might add a 50 ohm resistor.

Part of me tries hard to be nice to lab gear like coax cables.  The idea of 
deliberately cutting a cable in two just isn't part of my culture.  But it's a 
great way to get some coax already connected to a connector and it doesn't 
cost much.  (actually you get two of them)

That approach is often used for high speed scope connections.  Solder one end 
of a 1K resistor to the point you want to look at.  Attach center of coax to 
the other end.  Find a nearby ground.  Set the scope to 50 ohm input.  That 
gives you a 20:1 divider.  (actually, 21:1, use 950R if you want better 
accuracy)  Most TTL/CMOS drivers are happy with a 1K load.




-- 
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