When I received my SBIG ST-8 camera, there was a note with the camera describing problems users had experienced using the ST-7/8 and Pixcel 255 with Losmandy Mounts under battery power.  The subject of numerous posts to the SBIG mailing list, this problem seems to stem from grounding issues on the mount side, and SBIG offers a Relay Box for $65 to work around the problem (also apparently a problem with some Takahashi mounts) by isolating the circuits in the camera from the mount electronics.

Pedro Mardones created a very nice little circuit for optically isolating the camera from the mount, which has the same effect as the relay box.  His circuit has the additional benefit of allowing the hand controller and the camera to be used in parallel (the Losmandy drive unit has only one input that functions as either the hand controller input or CCD input).  Pedro's circuit involves making modifications to the Losmandy hand controller, however, and I didn't really want to do that.  So I borrowed heavily from Pedro's circuit, but put all the components into a little Radio Shack enclosure, which I velcro to the underside of the pier-pod base on the HGM-200 mount.  All external connections are RJ-11 type, so hookup is quick and foolproof.
 
 

 
 
 
 

Parts List

Qty	Part #	Description	Source	Total ($US)
4	4N37	Opto-Isolator IC (U1-U4 above)	Resource Electronics, Raleigh, NC	1.76
1	207-283A	Project enclosure w/drilled component board ("E" above)	Radio Shack	3.99
3	44-1009M-6	RJ-11 to DB-9M adaptors, 6-wire, assignable pinout ("C" above)	Capitol Electronics, Raleigh NC	2.59
10 ft	44-754-6	6-wire flat telephone cable	Capitol Electronics, Raleigh NC	1.50
2	???	6-pin RJ-11 crimp-on ends	Capitol Electronics, Raleigh NC	0.30
3	???	DB9F bulkhead connectors	Capitol Electronics, Raleigh NC	1.80
2	276-1993	2 pack 14-pin IC sockets, wire-wrap	Radio Shack	1.59
4	271-1113	330 Ohm .25 Watt resistors (R1-R4 above)	Radio Shack	0.49

Caveats:

1. The part marked "A" above comes with the camera and has a sticker on it that says "ST-7-RC".  The one I had didn't bring the +5V out to RJ-11 pin 6.  I found the wire inside the adapter shell and pushed the pin into the DB-9 connector pin 9.  This act has implications for using the "ST-7-RC" in other configurations, such as driving an Astro-Physics mount.  I would undo that change before plugging the cable into anything other than the opto-isolator circuit.
2. The part marked "B" is a cable I made up.  It is "flipped" because for some reason, SBIG chose to wire the signals into the RJ-11 on the ST-7-RC in backwards order (in comparison to the "standard" Telescope Interface Cable layout).  Just make sure this cable ONLY gets used for the connection from the camera to the opto-isolator box.  Mark it clearly "Camera".  It would probably be a good idea to look inside your ST-7-RC and verify how it is wired.
3. The part marked "C" is different from the "D" parts because it doesn't bring the unneeded ground connection into the DB-9.  Mark the shell on this adaptor "Camera" and everything should be fine.
4. The part marked "E" comes with a circuit board with a matrix of holes for fitting parts.  The hardest part of this project was chosing the location for and opening the 3 holes to attach the DB-9F bulkhead connectors.
5. If you are unsure which pin is pin 1 on an RJ-11 connector (I was), click here.  All the DB-9 connectors I've seen are clearly marked, but I need a magnifying glass and/or a bright light.
6. The connection from the opto-isolator box to the Losmandy mount is a cable I made up also.  It is a straight through RJ-11 plug 6 conductor cable (Pin1-to-Pin1, etc.).
7. If you want to know what the signals are, there are some pinout drawings in Appendix A of the SBIG manual.
8. TESTING: If you build it, I recommend a full end-to-end continuity check before you apply power.  This can save you from a world of heartache.  Next, test with just the hand controller to be sure all directions function.  The little LED on the hand controller should also light.  Then try the camera side.  If you have software that allows moving the scope, verify each direction with the software before you try calibration for guiding.
9. I make no claim as to the accuracy, function, usefullness, or damages caused by this circuit.  I offer no warranty.  It worked for me in my application.  That is all I can say.
10. If you see mistakes/omissions I would appreciate feedback (dlakey at mindspring dot com).   I would be happy to answer email questions about it if someone gets stuck.

Component Layout

This is just to give an idea of what it ended up looking like.  I had to be real careful with the clearance between the DB9F bulkhead connectors and the wiring on the components.  The thing velcro'd to the top is an unrelated RJ-45 through connector to let me extend the encoder cable on my digital setting circles.