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autoguider relay box would not operate unless pin 9 on the DB-9 connector plug was
grounded. Subsequent tests by Tom Sorbel confirmed that pin 9 has to be grounded on any
device plugged into the joystick and autoguide ports for it to be recognized as present by the
Compustar computer module. However, in the joystick port on the right side, a grounded pin 9
is also a signal to the Compustar computer module to inactivate the 4 motion control buttons.
This makes sense, as once the Compustar senses a joystick in the right hand port, it would
assume that you will then use the joystick to control movement of the mount. The Compustar
computer module “checks” to see if a joystick is connected to the right hand port (sensed by a
grounded pin 9) whenever the system is first booted up, or when the user hits the “ABORT”
button.
How Relays Work
Basically, a relay is an electrical switch. It is operated by electromagnetism. It has two
possible positions, which can be viewed as “on” and “off”, but these are normally referred to
as “closed” and “open”, respectively. That is, if the switch is making contact with one
position, then it is said that position is “closed”. If the switch is not making contact with a
position, then it is said that position is “open”. The relay switch has a default setting, which is
referred to as “normally closed”, often abbreviated as “nc”. A small spring holds the switch in
the “nc” position. Therefore, current normally flows through the switch via the “nc” position.
All the relays in the diagram are shown in their “nc” positions. A relay switch is in close
proximity to an electromagnetic coil, indicated by a circle with a “K” inside it. Because we
are controlling movement in 4 directions, there need to be 4 relay switches. Thus, the 4 coils in
these relays are marked as K1-K4. The 4 coils are drawn to the right of their corresponding
relay switch in the diagram. The relay switch is also labeled with its corresponding coil
number, such as K1, for example, by its “no” contact point.
If current is applied to the magnetic coil by an electrical input, like from one coming from the
autoguiding computer via one of the RJ-11 wires, then a magnetic field is produced in the coil.
This magnetic field will magnetically move the switch from its “nc” position to its “no”
position, causing the current to go through the circuit differently. When the electrical input to
the coil is stopped, then the magnetic field decays and there is no magnetic force holding it in
the “no” position, so the small spring flips it back to its “nc” position.
In the diagram, it can be seen that current coming from the autoguiding computer via the blue
RJ-11 number 6 wire (referred to as “operating +X” in the diagram) will induce an
electromagnetic field in the coil K1, adjacent to relay switch K1 (the uppermost relay on the
diagram). That electromagnetic field will switch relay 1 from its “nc” position to its “no”
position. That will have the effect of putting DB-9 pin number 1 of the Compustar guider port
at ground on DB-9 pin 5, 7 or 9 through the –X relay. This will upset the “balance” in
electrical forces on Pin 1 of the DB-9 connector in favor of the +X direction and move the
drive in the +X direction (this is extra movement beyond the normal clock drive movement).
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