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Tapping Serial output of USB GPS.

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:

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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:

??æææfÍææææ?ææææfÍææææM???3¦¦æ&æ?5gey=·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?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

Reply to
Kevin Walton
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  1. Have you tried other speeds? The "reported" speed may not be the speed the device is actually using.
  2. Have you checked the voltage level of the RXD signal? It could be OK to use with the USB converter chip but too low for your serial port.
  3. Does the program you're using expect any control leads (DSR, DCD, etc) to be active?
  4. If you're within driving distance of Atlanta, you can buy me a nice lunch and I'll dig out my serial line test hardware and software to determine exactly what signals the device is putting out.

More about me:

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VB3/VB6/C/PowerBasic source code:
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Freeware for the Palm with NS Basic source code:
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Drivers for Pablo graphics tablet and JamCam cameras:
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johnecarter at@at mindspring dot.dot com. Fix the obvious to reply by email.

Reply to
the Wiz

Check the Voltage Levels.

The PDF File at:

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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.

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Perhaps you need a voltage level converter:

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

Reply to
g9u5dd43_nospam

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

Reply to
Kevin Walton

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