Tapping Serial output of USB GPS.

[Kevin Walton]

Hi,

I'm doing some work with a GPS, see here for the full story:

sci.electronics.design subject: Cheap 433 Mhz Serial TX & RX - Which
ones!

It is a USB GPS but I need a serial output.

I have opened it up and it just uses a PL-2303 USB to Serial
converter:

http://tech.prolific.com.tw/visitor/fcabdl.asp?fid=17151518

with the SIRF 2e/LP GPS.

So I have tapped off of pin 5 (rx) and 7 (gnd) leaving the original
chip pins connected and connected to a PC serial port.

I can still operate the gps from the USB, it just uses a software
driver and pretends to be com6, connecting at 4800 8n1 I get my data
stream.

However, connecting to the serial tap at 4800 8n1 gets me data,
streamed at the right sort of rate and intervals - but it's
gobldegook. I have tried all combinations of 8/7, n/o/e and 1/2 at
4800 but no luck, just different gobledegook, this is the sort of
thing:

•šæææfÍææææÍšææææfÍææææMššš3¦¦æ&æÍš5gey=·â¾ââúN:">*>*F">>N>>>>F>>>>NÆN>>>>>F>>>>NêN>N>>N*>F>N>F>NÊN>F>NÊN>F>N>>>>V""ÊÖnâ¾â²úNúN:NNNNNNNNNNNNN*>F>N*>F>N*>F>V>*ÊÖn⾶ÊòN:">*>

Any pointers on how to fix the issue? Is there software that can tell
me what rate and format the data is coming in at? What should a scope
show me on the serial line? Am I using the right ground, I shouldn't
use the VDD_232 pin should I?

All ideas greatfuly received.

Thanks
Kev

 

[[email protected]]

Check the Voltage Levels.

The PDF File at:
http://tech.prolific.com.tw/visitor/fcabdl.asp?fid=17151518
States on page 9
Output Voltage, 3.3V*2:
*2. RS232 pins TXD, DTR_N, RTS_N are 3.3V tri-state outputs.

The RS232 states:
The region between +3 and -3 volts is undefined.
and you'll typically see +/- 12 volt levels on a PC.
http://www.beyondlogic.org/serial/serial.htm

Perhaps you need a voltage level converter:
http://www.superdroidrobots.com/rs232.htm


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[Kevin Walton]

Guys,

Thanks for the help. problem solved.

One MAX232 added after the serial tapping point and we have a nice
duplicate stream of serial data at 4800 8n1.

Now to wait for my 433Mhz modules to arrive

Cheers
Kev