|== Maxdome Card ==
|After suffering a failure of the dome control card we ordered a new card from Diffraction Limited. Upon installing it the dome immediately started to rotate endlessly. Pressing the stop button did stop the rotation, but only for as long as it was held in. Upon releasing the button, the dome began to rotate again.
|After some posts to the support forum, and some tests, we eventually figured out what was going on. Our old cards are Revision D. The new card is Revision F. Between those two revision, Diffraction Limited changed how the cards work.
|In Revision D, the motor+ and motor- terminals are held at +12V (so effectively 0V across them). When a rotation is commanded, one of the terminals drops to 0V, giving you a difference of 12V across the two terminals and engaging the motor.
|In Revision F they decided that holding the motor terminals at 12V is bad, just incase there’s a short somewhere. So now the terminals stay at 0V, and when a move is commanded the relevant terminal jumps to 12V.
|If you simply have a motor connected across the terminals then there’s effectively no difference. In revision D you have a difference of 0V until you command a move, at which point one of the terminals drops to 0 and you get you 12V difference. In Revision F they two are at 0 until a move is commanded, at which point one jumps to 12V.
|However. We use a large motor to drive our large dome, so we don’t/can’t drive the motor directly from the card. We instead use the voltage from the card to trigger our large external relays, which then engage the motor. And this was wired up in such a way to expect 12V on the motor terminals, with 0v commanding a move. This was accomplished by a jumper on one of the connection block connections (on the back left of the rotation box), between the 12V connection and the relay connection.
|Removing this jumper set the relays back to 0V and has them trigger when the MaxDome card motor terminals are at 12V (i.e. the new card behaviour).
|It’s important to be aware of this behaviour for the future. We returned our faulty card to be repaired, and so now have a spare Revision D card (in the computer room downstairs). We’ll have to be careful about the wiring if we use it to replace a card in the future.
|The dome shutter is still wired up to use a Revision D card, the rotation box is wired for Revision F.