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ETX-70, aux port and alignment issues

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Long story is below but to cut it short the auto-star wont align despite alot of trying so I wanted to see if I could upload new firmware and/or control the scope using the aux port via USB. The Weisner ETX site got me started but I'm now stuck and in need of some extra information that I can't find there.

I have connected and arduino up to the aux port and made a small PCB that pulls up the two CLK and DATA lines to 5V. The 9v pin is ignored but grounds are connected. 

Weisners site says that the AUX port uses an I2C interface but when I try to scan for I2C devices with the arduino the arduino crashes and occasionally the scope moves at random. I believe that it is not really an I2C interface and the data is more simplistic thus it is misreading some of the queries as instructions for movement. Has anyone ever dismantled a meade #506 cable or reverse engineered one?

If I can work out the protocol it should be possible to make a cable using an arduino or even an attiny so money terms less than a fiver and it would eliminate the need for a usb to serial adapter too. The info may actually be somewhere on Weisners site but its so badly organised I've spent a few hours hunting and still got nothing.


I have a small ETX-70 that is nice for some casual observing and I've set it up many times before and know all the basics.

It previously worked well with the two star alignment but the last few times I've set it up it complains that alignment stars that are below the horizon but they are definitely not. For example I was trying to use Arcturus two days ago at 8pm so it was definitely above the horizon approx 20 degrees.

I gave up and used Vega and the scope then missed it by a good 60 degrees or so. Has anyone else found this happening? I'm beginning to think the firmware no longer knows where things are as its too old. Any tips welcome! below is a list of what i've done already.

Level scope perfectly

Point north both with compass and using polaris

Train drives, calibrate motors

Double, triple, and quadruple check time date longitude and latitude

Use stars it allows in the setup (missed the second one by a mile and then fails when the second is centred)

Check battery (voltage 8.83v)


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