Digital I/O Features; 8254 Counter/Timer Circuitry - Keithley DAS-800 series User Manual

Digital I/O Features

8254 Counter/Timer Circuitry

DAS-800 Series boards contain three digital input lines and four digital
output lines. The digital input lines are associated with the IP1 / TRIG,
IP2, and IP3 pins on the main I/O connector; the digital output lines are
associated with the OP1, OP2, OP3, and OP4 pins on the main I/O
connector. Logic 1 at a pin indicates that the input/output is high (greater
than 2.0 V); logic 0 at a pin indicates that the input/output is low (less
than 0.8 V).
The digital input lines are compatible with TTL-level signals. If no signal
is connected to a digital input line, the input appears high (logic 1).
You can use the digital input and output lines for any general-purpose
task, with the following exceptions:
If you are using an expansion board for an analog input operation, the
four digital output lines control the multiplexers on the expansion
boards to determine the expansion board channel that is acquiring
data; in this case, you cannot use the digital output lines for
general-purpose digital output operations.
If you are using an external digital trigger or hardware gate, you must
use the IP1 / TRIG pin to attach the trigger/gate signal; in this case,
you cannot use IP1 / TRIG for general-purpose digital
input operations.
Each DAS-800 Series board contains 8254 counter/timer circuitry; the
8254 contains three counter/timers: C/T0, C/T1, and C/T2.
C/T0 is always available for general-purpose tasks. If you are using a
hardware internal clock source for an analog input operation, both C/T1
and C/T2 of the 8254 counter/timer circuitry are dedicated to internal
functions and cannot be used for general-purpose tasks. If you are using a
hardware external clock source, C/T0, C/T1, and C/T2 are always
available for general-purpose tasks.
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