USB adaptor (to RJ11)

This is an odd question, but I’ve got an odd problem. My GPS antenna [pick them up for under US$50 at Radioshack before they run out] came with cables that don’t work for my Palm TT. Before anyone asks, I’ve talked to Radioshack and custom adaptor suppliers, and there isn’t a cable to be had that doesn’t exceed the cost of the GPS.

But that’s OK: it looks like a simple problem on the surface. The GPS takes an RJ11 (phone) connector, so I know I need to transmit on pin 2 (TXD), receive on pin 3 (RXD), and ground on pin 5. So far so good. A universal palm adaptor, and I even have a spare, terminates in a USB. Hell, I even found an old female USB connector housing lying around my house.

The question is: how do I match up the 4 pins on the female USB connector to the 3 wires I need for the RJ11? It’s an easy solution to go from USB to DB9 or mini-din, but I can’t find out how to get to an RJ11. For grins I disassembled an old USB to mini-din converter (that’s how I got the female housing) and it was wired straight through – no resistors or jumpers that I could find. Based on that, I assume it is possible.

Are you sure the GPS signals on the RJ11 meet USB specs? It seems very unlikely to me as they would more probably use a USB or mini-USB connector.

sailor, no I don’t know that the signals would meet USB specs, but in this case the USB is only being used as a coupler. The signal is not going going into a USB port.

palm->16 pin->USB / USB->RJ11->GPS

The palm has the 16 pin to USB cable, the GPS has an RJ11 to ‘useless other device’ cable that I want to cut off and solder to a female USB.

Unless my logic is faulty, I know that 1) The palm works with the GPS when you have the proper cables, 2) the GPS works with a PC when you use a DB9 cable, and 3) DB9 can be converted to USB, so why not the RJ11?

TXD and RXD are not USB signals. I agree with sailor you will not get your GPS to connect over the computer via USB without some intervening hardware probably serial port to USB converter.

Unless you made the 16 pin palm to USB cable I think that it is a very good bet the palm expects to see a USB master on the other end of that cable. I don’t know specifically how your palm device converts the USB to 16 pin palm stuff but there are only two real choices.

  1. The USB signals route straight through the cable to some USB pins on the palm.

  2. There is a chip in the cable that is powered by the USB connection to the PC that converts USB to serial.

I am pretty sure that #2 is what is going on.

Well, I am not sure I follow you. DB9 means an RS232 serial port which is not the same as USB.

No, they aren’t the same, but you can use an adaptor to switch between the two.

gazpacho’s 2nd point is a good one. There might be device built into the cable. It might not be wired straight through.

A Serial to USB adapter is not a simple adapter. It has to handle the device-side (slave) USB logic to talk to the USB host. Your GPS antenna does not do that.
At first thought, it seems that you could get an RS232 to USB adapter (easy to find) and a DB9 to RJ11 adapter (also easy to find) to get data into your palm. However, neither your palm nor your adapter acts as a USB host, so they won’t be able to talk unless they implement USB2GO, which is too new so I doubt they do.

On the other hand, there are devices that talk to your palm device that I doubt handle the (relatively complex) USB host side. I would look into Palm SDKs and technical documents to see how it is done.

I think you were misunderstanding me because I was making the exact same point: The electrical signals are different. Clearly USB and DB9 connectors are different too.

Look at the end of this page. It has the pinout of the 16 pin palm connector. Don’t mess with the USB stuff and just hook up the rxd and txd to the correct pins. You proably connect the rxd pin of the GPS to the txd pin of the palm. I depends on if palm and the GPS thingy are dce or dte devices.

What a coincidence, gaspacho. I happened to find that same page yesterday evening. I didn’t bother with the USB after all. I rewired the 16 pin connector using that diagram to a 4 wire phone cable, and that should do it. Of course it didn’t work… but I’m assuming it’s a software problem at the moment.