I have GNSS device that outputs
I need to distribute both the RS232 and the PPS signal to more than 1 system (currently 3, but could be more in the future).
For one system (typical PC or embedded board) I need to even include the PPS also in the DB9 connector (PPS to the Carrier Detect pin). An indication of the architecture below:
Is there a cheap DIY solution for distributing (i.e. splitting) those two outputs and keeping signals "intact"?
- E.g. for distributing (or splitting) the RS232 I think a solution like this would be more than enough, right?
- What about the PPS signal? How should this one be distributed? I have found many commercial solutions (e.g. this) but those are expensive for our project.
Thank you!
Best Answer
I'm also looking into this for one of our projects. If you application needs accuracy down to a few 10s of nanoseconds, then you should probably be looking at 50 ohm drivers and be worrying about the propagation delays and overshoots. The commercial units will do this, but at considerable cost, and size.
For simple TTL signals, I would suggest something line this chip which would provide 8 outputs, or perhaps this one if you think you might have odd load capacitances etc.
Many devices seem to have a PPS input impedance of around 1k, so you might expect them to draw around 5mA at 5v. Also bear in mind that some devices generate very short PPS pulses. It can be useful to stretch the pulse with a monostable stage to make it easier to see on a scope, or a LED on a spare output..
Finally, have you tried just connecting the PPS pins directly? If the GPS can source 10-20mA, you might be ok without any driver/splitter.
I have also seen people wiring multiple RS232 Rx devices in parallel. I have never liked doing it, and it is not the 'correct' way of using RS232, but I'm sure there are several 1000s of systems using this solution as I type.
Interestingly, looking at this document They seem to use a 74HC14 Schmitt trigger chip as their commercial splitter, which is a nice and simple solution..