NMEA via RS232 and PPS (TTL) splitter

Question

I have GNSS device that outputs

• NMEA messages via RS232
• PPS signal via a digital out (TTL)

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"?

1. E.g. for distributing (or splitting) the RS232 I think a solution like this would be more than enough, right?
2. 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!

Answer 1

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..

Answer 2

Yes I've had to solve this too - not exactly for your scenario though.

I'm afraid the right DIY way to go about this would be to design your own PCB and put it in a box.

It actually sounds like a nice beginner PCB project. Pick the connectors that suit you, pick some chips (there are many chip models available, this is no rocket science), throw them all on a PCB, take care to block the chips' respective power input with suitable MLCC's, select a power supply (a +5V wall wart will do). Once you get some idea how large the PCB would have to be, try to get a suitable box for the PCB. There are PCB prototyping services that are willing to make and ship a small PCB for a couple USD worldwide... Try Kicad or Eagle Free to design the PCB.

For RS232, take a look at MAX232 and its equivalents. If you don't really need to level-shift the signals, it would probably be possible to repeat RS232 directly at the desired levels using some op-amp or comparator, ugly as it may feel...

If PCB design is not what you meant by DIY, the answer would be a little difficult :-) A passive split should work for 3 "consumer" / "line receiver" devices, but the chances of mischief are greater as you add more devices.

Meinberg have some in-house boxes (cigarette box size) for this very purpose, as an accessory to their timing GPS receivers - but the price tag is not quite DIY level.

Some vendors of "industrial control gadgets" have RS232 isolators and RS232/485 converters. These could be interesting to you if your wiring distances are non-trivial or if your local grounding is not perfectly even.