It is possible and fairly easy to control Synta telescope mounts (Skywatcher, Celestron, Skytech, and more) from a computer. The most common reason for doing this is to autoguide, which was also my motivation.
The mount interface is a standard serial device, but with TTL levels. TTL is an abbreviation for Transistor Transistor Levels, and means that a logical 1 = 5 volts and a logical 0 = 0 volts.
Normal serial devices usually work with something like logical 1 = -10 volts and logical 0 = +10 volts, thought there isn't a set standard. (Note: standard serial has inverted logic, on a hardware level a negativ value is logical 1.) If you fed your telescope with these levels you would probably damage the mount controller box, so we need a device that can convert between standard serial levels and TTL levels. This is what An EQ-Dir does, if you've heard that name before.
There are sites that sell these devices ready made, for bluetooth, USB, or a standard serial port, but they are very easy to make yourself, and cheap.
Since none of my current computers have a serial port, I had to find another way interface with my mount. I had two options here: a USB to serial adapter with a sort of TTL converter on the serial end, or a direct USB to TTL adapter. The USB to TTL is actually simpler to make, since the former would require a power supply to drive the TTL adapter.
You can a USB to TTL adapter from ebay, dx.com, or similar. Buy one with a chip from FTDI or Prolific, as these work well for this application. They cost as little as a US dollar including shipping.
The next, and only other, thing you'll need is an ethernet cable you can sacrifice. Cut it to desired length, and strip of a length of the outer insulation.
Only three wires are needed, one each for TX, RX, and signal ground. These are the minimum number of connections for a two-way serial interface. (Note:While researching the solution I came across a few posts that claim you also need to connect the RTS/CTS/DTR/DSR lines, but my mount worked happily without them. I almost never see these lines in use, and I suspect they can be left out on all Synta mounts.)
On the ethernet cable, it is the midle pins that are in use (4,5 & 6).
Connect as marked on the adapter, where 4 goes to GND, 5 to TX, and 6 to RX. The colours on the wires in the ethernet cable are normally blue (4), blue/white (5), and green (6).
That's it. The rest is all software.
