GPS cable

kenuk · Posted: Sat May 14, 2005 5:30 pm Post subject: GPS cable

One for the boffins.
I'am busy making up a Gps cable to go from my Ipaq 2110 to an
'Inforad' Gps Speed camera Locator it also doubles up as a Gps Receiver. the cable has a USB 'B' connection on one end and i'm putting a RJ11 plug on the other end ( same as the Tom Tom wired GPS receiver) i have the 'pin outs'

The question . the RX & TX wires. which one is Data + (plus) is it RX (recieve) or TX (transmit)
easy question.
answers on a post card or even better post in here Laughing Out Loud

Thanks.
Ken

lbendlin · Posted: Sat May 14, 2005 8:28 pm Post subject:

As long as you're not trying to directly connect a USB slave with a RS232 partner you can simply try both versions - there's no damage done when you have the wrong data lines connected - it just won't see the data)

Are you sure you have RS232 on both ends?
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kenuk · Posted: Sun May 15, 2005 12:54 am Post subject: GPS lead

The lead is USB type 'B' on one end going to a RJ-11 ( a small telephone plug type) as used on the tom tom wired unit. the usb end plugs into the Inforad unit and the telecom end into the Tom Tom / ipaq unit . all i have to do is put an RJ plug on i have the pin outs. ie: A1 is ground A2 is TX (DATA OUTPUT A3 RX DATA INPUT
A4 VCC POWER. The USB end is wired as standard ie: 1 = red 5 volt 2 = white data - ( neg) 3= green data + 4 =black ground

So. Data + = ? TX or RX

cheers
Ken

lbendlin · Posted: Sun May 15, 2005 2:56 am Post subject:

You CANNOT connect USB and RS232 directly.
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Pc-Mobile · Posted: Sun May 15, 2005 2:59 am Post subject:

As lbendlin replied, you can try to bot, swp it round itf it does not work.

If it is labelled or specified as data+ or -, it is related to USB only. It is not TX/RX.

Therefore it is very unlikey that you can make it work.
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Pc-Mobile
http://pc-mobile.net/gps.htm

kenuk · Posted: Sun May 15, 2005 2:41 pm Post subject: gps cable.

All USB Connections are marked that way.. Data + & Data -. or on some foriegn ones signal + signal -. and on the tom tom it is marked tx & rx on the RJ11 plug. i know which is + and - and obviously the middle two are the signal wires. i presume i only need one anyway.. the RX one as it is the receive.
thanks for your help anyway , i will try it both ways . and let you know the result
the company i bought the unit from advertise it (with optional cable for pda gps) but as yet they do not have them in. the unit is good and detection is first rate and update and obviously done by gps , and updates are regular.
have a look at the page.

http://www.inforadgps.com/
i actually bought it from the TheGadgetStore.com

lbendlin · Posted: Sun May 15, 2005 5:27 pm Post subject:

kenuk,

do NOT do that. It will fry your PDA. One last time: You MUST NOT attempt to connect a USB device directly with a RS232 (serial) device. They will NOT work together, and you WILL fry the RS232 part.
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kenuk · Posted: Sun May 15, 2005 8:15 pm Post subject: GPS CABLE

Ok, thanks got that . just have to wait until they supply me with the cable .
i wonder how they get round the problem . as the unit has only a usb connection. ? anyway thanks again

Ken

lbendlin · Posted: Sun May 15, 2005 9:12 pm Post subject:

most probably their cable contains a USB > Serial converter like for the Toshiba PDAs.
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Pc-Mobile · Posted: Mon May 16, 2005 4:30 am Post subject:

It is my understanding that the USB>serial converter is INSIDE the GPS.
I cannot understand why more and more GPS manufacturers are doing that. It is a long winded way to connect to outside world and there are more disadvantages than advantages.

If however, the USB>serial converter is in the cable then you will be in luck as you will be able to make a cable as you originally whished.
But I do not think that is the case.
_________________
Pc-Mobile
http://pc-mobile.net/gps.htm

Pc-Mobile · Posted: Mon May 16, 2005 4:34 am Post subject:

kenuk · Posted: Mon May 16, 2005 2:34 pm Post subject: GPS Cable

thanks for that bit info pc-mobile. i'm just waiting for a reply from Inforad then decide what next
cheers
Ken

BruceH · Occasional Visitor Joined: Jul 05, 2005 Posts: 4

:D
For those who have been trying to connect their GPS Inforad speed camera locator to a PDA this may help.

Firstly if you do not understand basic electronics, then this is not for you, as you will need a small number of components and a soldering iron.

I bought my Inforad because the original web page said that an optional cable to connect PDAs would be available. Last time I asked Inforad they said there were too many PDAs and the cable would not be available in the near future, so as an ex electronics engineer I had a look myself. This is not complicated so many of you should be able to get this made.

Firstly, the USB cable supplied by Inforad is SPECIAL. DO NOT MODIFY THE INFORAD SUPPLIED CABLE! The replacement costs about £15!
DO NOT USE A STANDARD USB CABLE TO CONNECT THE INFORAD TO A COMPUTER!!!

The cable contains a chip embedded in the large blob at the computer end of the cable. This chip converts serial data to USB, in conjunction with the drivers you load on the computer when you install Inforad Manager. I looked up the drivers and then found the manufacturers website, but you don’t need to know any more than the above.

To connect to your PDA use a standard USB cable, breakout the wiring and get access to the data lines. Look up the pin outs and cable idents on the web. Best not to cut the 0V and 5V as you will need to power the Inforad through these. Use the supplied cigarette adaptor to power the modified unit. May even power your PDA depending on power consumption, which is down to your judgement I accept no responsibility.

There is serial data in standard NMEA format coming out of the Inforad, but just to make life a little more complicated it is inverted. This is where you need some basic knowledge. Either build a simple inverter using a transistor and a few resistors, and yes, you will need a simple voltage divider to get the 5V output down to 3.3V to input to most PDAs.
I used a standard 74LS series Hex inverter and dropped the output to approx 3.3V. If you don’t understand the basics in the last paragraphs this is not for you! I don’t intend to go into detail here.

Anyway, once you have the serial data and inverted then connect to the serial port on your PDA. If using TomTom Navigator 3 or 5 configure as if you had a standard TomTom wired receiver.

This works perfectly on my Ipaq 2210 so hope it helps others

Cheers
BruceH

alweekes · Occasional Visitor Joined: Jul 09, 2005 Posts: 2

Hi all,

I just wanted to make a minor correction / clarification ot the above post.

As has been stated, the Inforad's USB cable contains a TTL-RS232 converter solution, that appears to the PC as a com port.

The USB type B socket on the inforad is not compliant to USB specs, since it has TTL (i.e. 0-5V) logic levels on it.

The data sense is inverted, relative to RS232 since this is the correct sense for TTL data.

Whilst you may, as the above poster has, be able to do a simple data inversion, this will NOT produce a true RS232 compliant interface. The RS232 interface is bipolar, i.e. it's voltage swings positive and negative, not just from 0V to a positive level.

The TIA/EIA-232-F specification says that the RS232 receiver threshold must be between +/-3V. In a perfect world, this means that the transmitters must swing a minimum of +/-3V. However, this leaves a 0V noise margin. Any noise that gets coupled onto the line could cause errors in the data. Therefore, the RS-232 specification says that the transmitters must swing a minimum of +/-5V. That said, most portable devices seem to be using +/-3V levels these days, and I suspect the receivers have higher sensitivities to give some noise margin.

To correctly interface the Inforad to an RS232 interface therefore requires the use of a TTL - RS232 converter solution. There are lots of IC's available (as free samples!) from Maxim (www.maxim-ic.com).

I've used a MAX3233 to add a compliant RS232 interface to my Inforad unit and it works a dream, if anyone is interested, I can post more details about how to do this.

The MAX3233 is a great solution, since it needs the minimum of external components and contains all the necessary circuitry to produce a negative voltage from the inforad's internal supplies. I've tapped off the +3.3V internal regulator to provide power for the chip, and at present have a DB9 connector fitted for a standard serial port, but am about to convert it to use with a car cradle and Tom Tom.

Regards,

Andy.

BruceH · Occasional Visitor Joined: Jul 05, 2005 Posts: 4

Never said it was RS232 with differential output, just thats its serial data, just like the output of most GPS devices, with conversion to USB. Data is inverted when compared to the standard output from a Garmin GPS12 or TomTom wired receiver. Inforad do use the USB connector in a non standard format, but 5V and 0V and data in /out comply with the standard pin outs.
My conversion uses one 74LS04 and a sub min 20K Ohm pot, can't get much less than that.
Like I said, I just kept it simple for those who have some knowledge.
If you want true RS232 do as alweekes says, just not needed for Ipaq, and I have yet to see a GPS unit with a true RS232 output.
Cheers

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