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Some more information I had saved in an old text file:
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Ground loops and power supplies are tricky things. Take these steps to find out if you have a possible ground loop.
1. Turn off all power supplies. Leave the power supplies plugged into the house current.
2. Disconnect all power supplies from the Gemini and camera. Leave the one to the computer plugged in.
3. Measure with an ohm meter between the Gemini power pins of the VCD OUT GND pin (see page 131 of L4 manual) on the Gemini to the Ground pin of the Camera and then a ground spot on the Computer. Also measure between the camera and computer. A reading of NO ohms would be perfect. Any low reading could mean you might have a ground loop. This means that the power supply input is not isolated from the power supply output.
4. Next turn all power supplies on. Measure for voltage between the ground outputs to each other. A zero reading is good. anything above 1 volt is bad. Voltage between 0 and 1 volt can also be bad, but depends on how large your wires are. Remember that you are doing this with no/very little current flowing.
5. Reconnect the camera, Gemini and computer. do step 4 with them connected. and see what you have. They should not have changed much. Again voltage between the units is a bad thing.
Computers can cause ground loops because they normally get connected to many other powered devices like monitors, networks and printers through wires.
The best way to eliminate ground loops is to have all the neutral (ground) DC output of every device tied to a central point, through a heavy (12 Gauge) wire. The central point when using a Gemini would be the Gemini ground input pins. This is not easy to alway do. The next best is opto or magnetic isolated inputs to the Guider and RS232 inputs.
If you do do a central ground between your devices, consider puting in a ground rod at least 8 ft deep next to the central point into the ground. This can help prevent static damage.
The best preventive is to use Isolated RS232 or USB adapters and Guiders into the Gemini. They can eliminate ground loops. I use one on all inputs to my Gemini. Of course I manufactured my own.
One word on note: The Gemini's with the slide switch for power do not have the electronics grounded to the case. The Gemini's with the Toggle switch do have the electronics grounded to the case.
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