Sharing problems

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Grant Privett

Thats interesting. As my previous problem had been using a USB sound sampler on a shared port, I thought I should check again.

So, I was out last night using a USB GPS dongle and NMEATIME2 to set my system clock to <0.005s accuracy, controlling a telescope from TheSkyX  (which uses a  USB to Serial converter to talk to a Paramount)  and a Starlight camera via a USB socket.

The laptop I used was a Dell E4310 (Intel i5 at 2.4GHz with 4GB memory) which only has 2 USB sockets.

I noticed that when anything was sharing a socket with the dongle, the NMEATIME2 correction timings became erratic – it assumes GPS  fixes from the receiver are evenly spaced I think. So, when I connected the telescope or when sharing with the CCD my timing precision was potentially off as the attached image demonstrates (the sharp change in the trace is when I connected the telescope). Not surprising I suppose, but the timesharing aspect clearly does have some an effect on other bus users. I imagine the impact will depend upon the nature of whats sharing the port and what laptop manufacturer/model you choose.

I will try again tonight if (wonder of wonders) we get two clear nights in a row using a Dell D630 which has 4 USB ports and post the results here. Alas, I only have Dell laptops and so cannot say how this pans out with other machines.