Counter/Timer Subsystem; Counter/Timer Initialization; Counter/Timer Interrupt - Keithley DAS-1800ST Series User Manual

Counter/Timer Subsystem

The DAS-1800 uses an Intel 8254 Programmable Interval Timer that
consists of 3 internal 16-bit counters, Counter 0, Counter 1, and Counter
2. For A/D pacing, the DAS-1800 boards operate Counters 1 and 2 in a
fixed divider, 32-bit configuration. The input of Counter 1 connects to a
5MHz crystal oscillator and the output of Counter 1 connects to the input
of Counter 2. The DAS-1800 uses Counter 0 for trigger delay and its
input, output, and gate control are not available.
Although the DAS-1800 uses a chip that has 3 counters, it ties two of
them together internally (Counters 1 and 2) and dedicates the third to
internal functions. Therefore, DriverLINX implements only one logical
counter, Logical counter 0, which corresponds to the internal A/D pacing
clock (8254 counters 1 and 2). You can use Logical counter 0 to pace
Analog Output or Digital I/O tasks indirectly through hardware
interrupts. The DAS-1800AO has an additional counter/timer for
supporting timed Analog Output tasks in hardware. DriverLINX
implements two logical counters in this case, Logical counter 0, which
corresponds to the internal A/D pacing clock, and logical counter 1,
which corresponds to the internal D/A pacing clock.

Counter/Timer Initialization

Power-up state of the counters is undefined. Initialization establishes the
state of each counter as a binary rate generator with a down count of
65535.

Counter/Timer Interrupt

DriverLINX supports counter/timer interrupts indirectly. You can set up
an INTERRUPT mode A/D task and use the interrupts to pace the
counter/timer task.
Counter/Timer Subsystem
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