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This is sort of a follow up to my previous question regarding power supply for my telescope. The general advice I got was that I should just ditch regulation and connect the lead-acid battery directly to the telescope and camera. I have read the manuals for both these devices and both state that is is safe to do so as they both have internal regulators anyway.

However, I almost always connect my telescope to the computer via RS232. My computer does not have a RS232 port, so I'm using a Keyspan USB to RS232 adapter. I don't think the adapter is isolated because I checked with my DVM and there is continuity between the grounds of both (that is the USB and RS232).

The problem is my telescope's electronics are known to be problematic when it comes to ground loops. Some people have outright blown the RS232 chip due to this.

My question is, since the computer is going to be connected to the mains and the telescope + camera to the 12V battery, there is no common ground. Am I going to need an RS232 opto-isolater for the telescope->computer connection?

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If your telescope supply is from a battery, it is already isolated from ground; when you connect the 232 cable, its system will become referenced to that ground.

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  • \$\begingroup\$ Do you mean the ground from the USB i.e. computer? Couldn't this be different from the return (or -ve) of the 12V battery? \$\endgroup\$ – Saad May 4 '11 at 11:27
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    \$\begingroup\$ @saad, negative of a battery is not ground. Unless you connect it to ground yourself. \$\endgroup\$ – Martin May 4 '11 at 12:21
  • \$\begingroup\$ +1 what Martin said - the battery, and any load connected to it, are 'floating' with respect to ground, unless you establish a connection between some part of the battery/load system and an actual ground. In this case, you can take the ground in your 232 connection as the definition of 'ground' for your battery/load combo, once you connect it. \$\endgroup\$ – JustJeff May 4 '11 at 22:17
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I would have not guessed this would be a problem (and the issue probably would not be ground loops per se) but if other people have blown connected electronics, I suppose it pays to be careful.

If your baud rate isn't too high you should be able to make your own RS232 opto isolation pretty simply.

Another option would be to use an embedded Ethernet to rs232 gateway, since UTP (ie CAT 5) Ethernet interfaces are usually transformer isolated (at least if you aren't doing power-over-ethernet). I'm seeing these in the neighborhood of $30, and they'd let you run cat 5 cable out to the telescope.

Another option would be to go wireless...

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