[LinuxPPS] Some Info about the Project I'm working on

[Rodolfo Giometti]

On Fri, Sep 07, 2007 at 03:36:24PM -0500, Adam Becker wrote:
> Thank you all for your input. 
> 
> Hopefully the following will help clear up how the system is configured:
> 1) PPS - CPU's 1, 2, and 3 receive PPS via I/O pins on their parallel
> port

Ok, in this case you need the LinuxPPS's parallel support.

> 2) GPS - Only CPU 1 receives via COM1. 
> 3) Processor Communications
> 	a) CPU 1 - COM2 to  CPU 2 - COM1
> 	b) CPU 1 - COM3 to  CPU 3 - COM1

So you should duplicate GPS data for CPU 2 and 3, maybe using the tee
command and named pipes:

   $ mknod p pipe1
   $ mknod p pipe2
   $ tee pipe1 < /dev/ttyS0 > /dev/ttyS1
   $ tee pipe2 < pipe1 > /dev/ttyS2

Then CPU1 can read PPS data from pipe2. 

> 4) Accurate - As accurate as possible
>
> I really like the idea of using ethernet to sync the clocks. To do so
> I'm thinking I'll incorporate a switch into the system. However, I'm
> concerned about how reliable messages sent using ethernet are. Isn't
> their some lag time and acknowledgment issues sending/receiving data via
> ethernet?

Yes, but NTP protocol is designed to keep in account these delays (see
relative documentation). However to get best results maybe your
current solution is ok (even if you should do some tests before
discarding my solution :).

Ciao,

Rodolfo

-- 

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