ADLINK PCI-8124-C - Manual (Page 42)

32 Operation Theory
4.1.6 FIFO
FIFO is first-in-first-out storage. It is used for storing some preset
position data for comparing. Every time the position is compared,
a new data is retrieve from FIFO into comparator. This mechanism
makes a the continuous triggering function.
4.1.7 PWM & Mapping
PWM is used for adjusting pulse width of trigger. It could also be
switched to a toggle mode. In this mode, the pulse level will
change from low to high or high to low at every time when com-
pared.
Mapping means four trigger signals are not one-to-one mapping to
four comparators. For example, Comparator 1 could be linked to
trigger channel 2. Comparator 2 could be linked to trigger channel
1 and 4. Comparator 2 and 4 could be linked to channel 3.
4.1.8 Digital Filter
The digital filter is used to filter noise behind the encoder input cir-
cuit. The encoder signals are typically disturbed with environmen-
tal noise, and such noise affects the accuracy of the position count
which is generated by calculating the encoder input pulse quantity.
The position counter will increase or decrease because of the
noise blending with the normal encoder signals. In this card, sig-
nals over 5 MHz will be filtered which means that the signals are
regarded as noise if its frequency is over 5 MHz. For instance, if
the input signal frequency is 2 MHz and accompanied with 8 MHz
noise coupling, after filtered, the 2 MHz will be reserved for the
decoding process.
4.1.9 Trigger Counter
In general, the accuracy of trigger pulse output quantity may be
misjudged because the incorrect image retrieved. During a line
scan process, the frame grabber will trigger the CCD capturing the
line image after receiving the trigger signal from the encoder
board. Therefore, if the encoder does not properly issue a trigger
pulse to frame grabber, then the incorrect image may be retrieved.
However, there are many possibilities will may cause this problem,