The motion- and color-related signals are arranged as differential
pairs. For the purpose of the laser projector, the word “differential”
means that the laser projector must derive the actual signal level by
taking the difference between two signals (i.e. by subtracting). For
example, ILDA DB-25 pin 1 contains the X+ signal, and pin 14
contains the X- signal. Often times, when pin 1 is going from 0V to
+5V, pin 14 will be going from 0V to –5V. The actual signal level is
found by subtracting; thus +5V minus –5V = +10V. This means that
the voltage level that the X scanner amp should sense is +10V.
Note that I used the term “often times” above. The reality is that there
is no strict requirement for the X- signal to be “equal but opposite”
when compared to the X+ signal. As far as the projector is concerned,
the same X position could be commanded if the X+ signal goes to
+10V and the X- signal stays at 0V, because +10V minus 0V = +10V.
Likewise, the same result could be generated if the X+ signal goes to
+20V and the X- signal goes to +10V, because +20V minus +10V =
+10V.
This is an important concept, because the ISP standard absolutely
requires projectors to derive all motion and color signals by taking the
difference between two signals. No motion or color signal should be
assumed to be referenced to ground, and – within the projector – no
motion or color signal should be connected to ground.
With that in mind, you will notice that we connect ILDA DB25 pin 1
to the X+ input on the X scanner amplifier, and we connect pin 14 to
the X- input.
For the Y axis, we connect pin 2 to the Y+ input of the Y scanner
amplifier, and we connect pin 15 to the Y- input.
Note that, although the scanner amplifiers themselves may have a
“Signal Ground” input (labeled SG in the diagram), this is NOT
connected!! The reason this is not connected is because if it were, this
would destroy the single-point grounding scheme that was established
and discussed above in the Power Connections section of this article.
Basically, since the scanner amplifiers have differential inputs, it is
only those differential inputs that are used for ILDA DB-25 signals.
The only ground connection is made from the “Power Ground” to the
“Central Grounding Point” already mentioned above.
If a scan-fail interlock were used, it would be connected to the X
POSITION and Y POSITION signal from the scanner amplifier. But,
there is something tricky to watch out for. You should consult the
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