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:
RJ45-DB9 adapter, P/N
66190. Similar adapters are common, but if you use the Jameco
product the wire colors will match these instructions.
- 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.
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.
- Heat-shrink tubing or
vinyl electrical tape.
- 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
- +18V (possibly +12V
on the GPS models, I haven't looked at this)
- Misc TX (not used)
- PC RX
- PC TX
- 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:
And here's the procedure
to build the adapter: