GPS Problem

aswilli · Joined: 21 Mar 2006 Posts: 7 Location: Las Cruces, NM

I have a Garmin 16 LVS system. I am having problems with it now when I am trying to read it using a UART on a PIC microcontroller. I am hooking it up though a MAX 232 chip in order to create the proper levels between the PIC and the GPS unit. I am finding that the GPS puts out a nice square wave when unloaded and purely viewed on an O-scope. It also transmitts to hyperterminal just fine. However when I try to hook it up to the MAX 232 chip, there seems to be some sort of impedance miss match and I don't get the clean square waves anymore. The amplitude decreases and the waves turn more triangular. It does this badly enough that the MAX does not know how to pass this information on to the PIC in the form of a 5V set of pulses. Anyone else ever ran into this sort of thing? Any help you could provide to help me remedy this problem would be much appreciated. Thanks

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Here is the spec for the Garmin unit:
http://www.garmin.com/manuals/GPS16HVS_TechnicalSpecifications.pdf

You said it works OK when connected to a PC. The PC's RS-232 level
translator is going to be effectively the same as a MAX232, in terms
of the load presented to the Garmin unit. Therefore I think you have
a wiring problem with your MAX232 chip. Check all connections,
component values, and power and ground.

Guest · Posted: Wed Apr 26, 2006 4:35 pm

It is a pre setup MAX chip on a Demo board. It works fine when a regular PC input is applied at this point. However I have both a GPS and a digital compass that cause this sort of signal degredation on the UART. I am very confused by this whole problem. Any ideas?

aswilli · Joined: 21 Mar 2006 Posts: 7 Location: Las Cruces, NM

It is a pre setup MAX chip on a Demo board. It works fine when a regular PC input is applied at this point. However I have both a GPS and a digital compass that cause this sort of signal degredation on the UART. I am very confused by this whole problem. Any ideas?

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

OK, I think this is the "Null modem" cable problem that comes up
fairly often on this board. The PC has a RS-232 connector in the
"DTE" configuration. Your Demo board and the Garmin are in "DCE"
configuration. Each can talk to the PC with no problems. But they
cannot talk to each other. The solution is to get a cross-over cable
also known as a "Null modem" cable. Instead of being wired "straight
through", this cable has pin 2 on one end going to pin 3 on the other
end. And also pin 3 on the first end goes to pin 2 on the other end.
i.e., the wires for pins 2 and 3 are swapped. You still need pin 5
connected to pin 5 for a common ground.

Do a web search for: null modem cable pinout

aswilli · Joined: 21 Mar 2006 Posts: 7 Location: Las Cruces, NM

Thanks much. I found this out just before I read your newest post. Thanks for your help. Works like a champ now.