Parallax GPS Module communication difficulty

jwt2368 · 2010-05-10 05:32

I am attempting to communicate with the Parallax GPS module using standard RS-232 protocol with the following parameters:
Baud rate=4800 (pulse width = 200usec)
+5V TTL with RAW pin left open (smart mode)

I tried sending a message command such as "!GPS02" and I do not receive a response from the module.
For development purposes, I have connected the RS232 TX pin of a PIC to the SIO via a transistor and left the RX pin disconnected. (Reference the schematic attachment)
I then have an oscilloscope that monitors the SIO pin for a response from the module.
The "!GPS02" command is sent to the module but there is no reply from the GPS module.
The module seems to be working correctly. It transmits raw NMEA strings and the LED is on/blinks etc.

I cannot find a document that illustrates the AppMod serial protocol at the bit level so i am not sure if my start and stop bits are what the module expects.
All i have been able to find is the Basic code that implements this protocol...
I also included a simple "01" transmission so the start and stop bits can be more easily seen than if I uploaded the entire "!GPS02" transmission.

I would like help in verifying that the start/stop bits and the bit-stream format is correct or if there exist a document specifying AppMod protocol.
Note that on the schematic the RS-232 comes in inverted and is inverted again with the 2N3904 before reaching the SIO.

Thanks in advance,
JT

hover1 · 2010-05-10 11:42

This is a quote from a Nuts and Volts article, Column #139: Hacking the Parallax GPS Module.

"Some time back Parallax developed a simple serial communications protocol for its

AppMods; devices that connected to the BOE’s AppMod header. The protocol uses Open-

True serial mode – this is critical. True mode means that the idle state of the serial line is

high, and Open True means that the idle (0) state is accomplished with a pull-up. When a

transmitting device wants to exert a "1" bit it pulls the line low. This configuration allows

several devices to be bussed together without fear of electrical conflicts as the serial line is

never driven high.

By tradition, the AppMod protocol starts with a "!" character, followed by a two- or threecharacter

device ID string. The original intent of the "!" character was to allow devices to

determine the baud rate of the incoming stream. We’re not going to do that here as auto-baud

programming can be quite messy, and our goal is to have fun. Since the likely host of the

GPS module will be a BS2-type processor, we’re going to fix the baud rate to 38.4K; this

matches the high-speed mode of the PSC, and allows the host program to have a single baud

constant for both devices."

There may be other information in the article that may help.

Jim

Post Edited (hover1) : 5/10/2010 11:49:10 AM GMT

Chris Savage · 2010-05-11 20:09

Hello, you're sending a string of "$GPS02", however the command should be "$GPS", $02. You need to send the value (2) out after the $GPS string, but you're converting the command value to a 2-digit string. Send the ASCII string followed by a single byte command value and then be ready to receive the return data on the same I/O pin.· I hope this helps.

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Chris Savage

Parallax Engineering
·

hover1 · 2010-05-11 20:15

Nice catch Chris. I was thinking it was a problem with protocol, so I didn't look at the syntax.

Jim

Chris Savage · 2010-05-11 20:22

Well, I'm not sure if that is being followed either...the OP mentioned RS-232 TX pin and RX disconnected. That's not going to work. So he should read that post as well.

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Chris Savage

Parallax Engineering
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Parallax GPS Module communication difficulty

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