Unable to read GPS data via /dev/ttyS3

[Maarten van Berkel]

I am trying to read NMEA messages from the GPS module via the serial console using the following command:

cat /dev/ttyS3

In a different shell, I run venus-gps without any commands, as described in http://www.multitech.net/developer/products/multiconnect-ocg/applications/venus-gps-package/

When I start venus-gps, I read a few ‘#’ from /dev/ttyS3, but not the NMEA info I expected.

Few questions:

* how can I see if the GPS has a fix (e.g. via a LED?)

* how can I programmatically determine whether the GPS is properly initialized and has a fix?

* is it correct that I need to run venus-gps as a daemon in order to read the data from /dev/ttyS3?

* is the correct way to read this data cat /dev/ttyS3?

Any help is appreciated!

[Jesse Gilles]

> is the correct way to read this data cat /dev/ttyS3?

You should be able to read data that way, yes. You can also use ‘microcom’ — it usually works better and it is better for doing serial-specific communication because you can set baud rate, etc. The GPS transmits at 9600 baud and microcom also defaults to 9600, so you should be able to run ‘microcom /dev/ttyS3’ and see the NMEA data coming out.

> is it correct that I need to run venus-gps as a daemon in order to read the data from /dev/ttyS3?

No, you don’t need to run venus-gps at all to read GPS data from the receiver. Just open /dev/ttyS3 at 9600 baud. If you’re having issues, I would try power cycling the unit, and then running ‘microcom /dev/ttyS3’ after it boots up. This should work or something is wrong. You only need to use venus-gps if you want to change settings on the GPS receiver or if you want to send the data to a TCP/UDP socket, etc. You should only have one program at a time reading data from /dev/ttyS3 — either venus-gps or microcom, or a piece of custom software, but not more than one.

> how can I programmatically determine whether the GPS is properly initialized and has a fix?

Once you can read data from /dev/ttyS3, you can decode the NMEA data to see if the receiver has a fix or not. There is a link to a NMEA document on the References page

http://www.multitech.net/developer/products/multiconnect-ocg/references/

General info on talking to the receiver:

http://www.multitech.net/developer/products/multiconnect-ocg/applications/venus-gps-receiver/

> how can I see if the GPS has a fix (e.g. via a LED?)

There is an LED on the GPS board that shows when it has a fix, but you have to remove the case to see it. You’ll be able to see which board it is by looking at where the GPS antenna cable is connected. The LED flashes when it has a fix and it’s solid when it does not.

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