Cable to Connect an LX200 to a Dell Axim

The Dell Axim is a very nice PDA, low cost and with good performance. It provides a nice platform for TheSky Pocket Edition by Software Bisque. This program is capable of controlling an LX200, but the serial cable provided by Dell is designed to attach to a connector implementing the full RS232 standard, not the 3-wire subset used by the LX200 and most other telescopes. The problem is that the Axim serial cable contains a circuit that performs the required level shifting, and it expects to see power on pins that aren't available in a 3-wire implementation. If the correct levels aren't seen, the circuit shuts itself off to save power. All the methods of powering the cable involving tying certain pins to other pins seem very unreliable. For instance, if the Axim shuts itself off after a period of activity, a restart may not get the cable working. Fortunately, the LX200 serial interface provides a powered output that can be used to fool the cable.

To do this, you will need the following materials:

  1. Jameco RJ45-DB9 adapter, P/N 66190. Similar adapters are common, but if you use the Jameco product the wire colors will match these instructions.
  2. 6-conductor flat phone cable terminated with connectors. You can buy one of these already made, or make one with parts from Radio Shack, Home Depot, or many other places. If you use a pre-made cable, make sure it is wired straight through, or be sure that you are connecting the correct wires in the adapter.
  3. Dell serial sync cable. Yes, they really make one for the Axim, but it is hard to find on their website. I had to call to order one, 800-917-3355. About $20.
  4. Heat-shrink tubing or vinyl electrical tape.
  5. Wire cutters, wire strippers, solder and soldering iron.

A word on the LX200 connector wiring: The Meade documentation is very confusing, and is perhaps incorrect. Ignore it. Instead, just number the pins on the scope's RS232 connector sequentially from left to right as 1 through 6. If you do this, the pin assignments are

  1. +18V (possibly +12V on the GPS models, I haven't looked at this)
  2. Common
  3. Misc TX (not used)
  4. PC RX
  5. PC TX
  6. Misc RX (not used)

We can't just use the 18V signal to fool the cable because that is higher than the maximum voltage the cable will tolerate. So the purpose of this cable adapter is to reduce the voltage. Using the above pin numbering scheme, here's the schematic:

Axim Adapter Schematic

And here's the procedure to build the adapter:
Step 1Begin by cutting off all the wires in the adapter except for the four that are used. If you are using the Jameco part, you only want to leave the brown, yellow, red, and black wires. Click to see a larger image.
Step 2Cut the end of the brown wire off, and strip and tin the two ends. Strip a small middle section of the yellow wire and tin it. Click to see a larger image or detail image.
Step 3Put a short piece of heat-shrink tubing on the brown wire where it comes out of the adapter and push it away from the stripped end. Connect the two resistors together and solder. Add the loose brown wire segment to this junction. Click to see a larger image or detail image.
Step 4 Attach one of the resistor ends to the brown wire, and the other to the middle of the yellow wire. Click to see a larger image or detail image.
Step 5 Slide the heat-shrink tubing over the joint between the brown wire and the resistor, and shrink it. Click to see a larger image or detail image.
Step 6 Put a larger piece of heat-shrink tubing over the entire resistor assembly, and shrink it. You can also use vinyl tape for insulation. Click to see a larger image or detail image.
Step 7 Insert the connector pins into the connector shell. Brown to pin 4, yellow to pin 5, red to pin 3, and black to pin 2. Push the connector into the adapter housing. You might want to label the assembly with a permanent marker, since these things all look the same once they are assembled. Click to see a larger image or detail image.
Axim System If necessary, make a short cable from 6-conductor flat phone wire. Click to see a larger image.
Axim System You are ready to connect your Axim to your telescope. The order you start TheSky or plug in the cable doesn't matter. If the Axim offers to start a sync when you attach it to the scope or the cable, just hit Cancel. Click to see a larger image.
© Copyright 2003, Chris L Peterson. All rights reserved.