[ntp:questions] OT: GPS18x LVC failure

David J Taylor david-taylor at blueyonder.not-this-part.nor-this.co.uk.invalid
Mon Oct 12 06:51:04 UTC 2009


"David Lord" <> wrote in message 
news:7jen4sF32s09iU1 at mid.individual.net...
[]
> The pps via usb doesn't work with NetBSD. That is I believe lack
> of hardware interrupt capability via usb. Possibly latest
> development pps might work from usb on Linux.

It certainly works on Windows.  Is that because Windows does a better job 
of emulating the interrupts than NetBSD does, or because of the 
architecture which Dave Hart developed for Windows does a better job of 
picking up the timing, or because I was lucky with my USB device or its 
drivers?

> Problem I'm seeing is good ttl inputs (< +300mV) to usb converter
> not producing any output at pc whilst
>
> (1) ok using modem connected to serial input of converters. AT
> commands via cu work ok.
>
> (2) some erratic recogniable output with ttl input to converter
> (PL2303), no output with ttl input to converter (FTDI).
>
> (3) ttl outputs connected to serial inputs on server work ok
> from gps18x-lvc+pps (2us)
>
> (4) ttl outputs possibly a bit erratic for pps from Conrad MSF
> module via diy pps extractor as only 25% of polls with reach 377,
> but possibly this is to be expected. MSF via dsr or dcd almost
> 100% reach 377.

Agreed it sounds like levels.

> So usb to pc is ok and it does appears my ttl levels at converter
> inputs aren't acceptable. I expect output +9v to -9V via a 1488
> will be ok. I've not set that up yet as intended generating
> +/-10V using switched capacitor converters I ordered, but these
> didn't arrive and went from "in stock" to "no longer available"
> overnight between my order being accepted and delivery of
> remainder of order. I'll try this week with 1488 on +/-5V and
> higher from bench psu.

Tiresome about the chips!

[]
> Gerhard Dangl's astronomy equipment
> <http://www.dangl.at/ausruest/gps18lvc_e.htm>

Fascinating page.....

> above page shows scope traces -5V to +5V swings from gps18x-lvc.
> Spec states 0V to +5V and data sheet I have is same as latest on
> Garmin site.
>
> I have pico and velleman pc scopes but rarely use them. I might
> try capturing some waveforms from my kit.

If the Garmin can deliver +/-5V, it should certainly drive almost any 
reasonable RS-232 port.  It sounds as if your USB converter has a lower 
input resistance than the Garmin is expecting, or has too high a 
hysteresis.  I can't recall offhand what the specification is for RS-232 - 
a few K-ohms is what I recall.

If you want to see inside the older GPS 18 LVC:
  http://geoff36.googlepages.com/insidegarmin18lvcgps

Cheers,
David 




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