offers an additional layer of safety for the laser projector, and thus it is
a desirable thing to have.
Unlike the motion control signals, and color control signals, the
shutter signal is not considered to be an “analog” signal. It is TTL in
nature, and thus, the shutter is either fully opened, or fully closed. The
shutter is fully opened when pin 13 is roughly 5V when compared to
pin 25. Also, the shutter signal is not a true differential signal, since
noise immunity is not really needed, due to the fact that it is TTL in
nature. However, for the purpose of projector connections, it should
be considered to be a differential signal whose counterpart is pin 25.
ILDA DB25 pin 25 is considered to be the “Common” signal of the
ILDA connector. However, this “Common” signal is not necessarily a
“Ground” signal, since, under many circumstances, this signal is not
connected to “Ground” within the projector.
Since the shutter signal is TTL in nature, and referenced to DB-25 pin
25, one good way to receive this signal is using an optical isolator.
The optical isolator will receive the TTL-level signal between pin 13
and pin 25 and allow isolation of these signals from the rest of the
projector components.
DB-25 Connectors
For projectors, the ISP specification states that the DB-25 connector
should be a male connector. However, I recommend putting two
connectors on the projector – one male, and one female. This
maximizes ease-of-use because it allows the use of any type of DB-25
cable between the projector and signal source, and also allows easy
daisy-chaining of multiple projectors when needed.
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