INPUT POLARITY JUMPERS
Two jumpers located on
the printed wiring board (PWB) separately select the
desired input polarities for inputs In A and In B. P =
positive, N = negative.
A COUNTER/TIMER JUMPER
Two-position jumper
located on the PWB. In the Counter position, Counter A
always counts and displays the events connected to In
A. When set to the Timer position, Counter A counts and
displays the time if either the 0.01-Sec or the 0.01-Min
time base is selected. If the Ext time base is selected,
Counter A will count and display the events from In A.
B COUNTER/TIMER JUMPER
Two-position jumper
located on the PWB. In the Counter position, Counter B
always counts and displays the events from In B. In the
Timer position with the Ext time base selected, Counter
B counts and displays the time in 0.01-second intervals.
With either a 0.01-Sec or 0.01-Min time base selected,
Counter B counts and displays the events from In B.
GATE A (LIVE TIME/NORMAL) JUMPER
Two-position
jumper mounted on the PWB. In the Normal position, the
signals from the rear-panel Gate A connector gate the
events from the In A connector. In the Live Time position,
the signals from the Gate A connector gate the 10-MHz
clock to form a live-time clock.
1 CYCLE/RECYCLE
Selection of either the 1 Cycle or
the Recycle mode can be made via an 8-pin DIP switch
on the RS-232-C interface board. The Recycle mode can
be used when the computer is able to respond with a
data transfer when the Model 994 reaches the preset
value. Upon reaching preset, the Model 994 latches its
data into a buffer, resets the counters, and starts the
next counting interval. This process takes ~50 µs. The
computer reads the data in the buffer before the next
counting interval ends. In the 1 Cycle mode, the Model
994 simply stops counting and waits for further
commands when the preset value is reached.
INPUTS
IN A
Use of this input is affected by the A Counter/Timer
Jumper.
Positive Input
Front-panel BNC connector for Counter
A accepts positive unipolar signals; minimum width
above threshold, 20 ns at a 50% duty cycle. The
threshold is adjustable from +100 mV to +9.5 V via a
front-panel 25-turn trimpot. Z
in
= 1000
to ground;
dc-coupled.
Negative Input
Changing the Input Polarity Jumper
position on the counter board permits selection of the
NIM-standard fast-negative logic input which is designed
to accept –600 to –1800 mV pulses with a fixed
discriminator threshold of –250 mV. Z
in
= 50
; dc-
coupled. Minimum pulse width above threshold is 4 ns.
IN B
Identical to In A except that it feeds Counter B. Use
of this input is affected by the B Counter/Timer Jumper.
ENABLE
Front-panel BNC input connector accepts
NIM-standard, slow-positive logic pulses to control the
counting condition of the entire module. A level of >+3 V
or open circuit allows counting provided the instrument is
in the Count mode and has not reached the preset
value; <+1.5 V inhibits counting. The driving source
must be capable of sinking 5 mA of positive current
during inhibit; input protected to +25 V.
GATE A
Rear-panel BNC input connector is identical to
the Gate B input with the following exception. With the
Gate A jumper on the PWB set to the Normal position,
the Gate A input controls counting of the In A events in
Counter A. By moving the PWB Gate A jumper to the
Live Time position, the Gate A input also controls the
10-MHz clock to form a live-time clock with a 100-ns
resolution. A level >+3 V or an open circuit allows
counting of the clock. A level <+1.5 V is used to inhibit
counting of the clock during dead-time intervals.
GATE B
Rear-panel BNC connector accepts NIM-
standard, slow-positive logic signals to control the
counting in Counter B. A level >+3 V or open circuit
allows counting; <+1.5 V inhibits counting; input
protected to +25 V. The driving source must be capable
of sinking 5 mA of positive current during inhibit.
OUTPUTS
INTERVAL
Front-panel output BNC connector furnishes
a positive level during the counting interval. The level is
nom5 V when counting is enabled and <+0.5 V
when counting is disabled. Z
o
~30
.
OVFL A
Rear-panel output BNC connector provides a
NIM-standard, slow-positive logic signal each time
Counter A overflows its 8-decade capacity. The signal
has a nominal amplitude of +5 V; width ~20 µs.
OVFL B
Rear-panel output identical to OVFLA except it
monitors overflows from Counter B.
INTERFACE
SERIAL
When the RS-232-C option board is plugged in,
it furnishes a rear-panel, 25-pin, male, D connector
containing all signals for standard RS-232-C
communications. It also contains connections for 20-mA
current loop communications. The field-installable
RS-232-C option provides computer control of the
following functions: Count, Stop, Reset, Remote, setting
the preset value, selecting the displayed counter, and
selecting the desired time base. In the Remote mode,
the computer can disable all front-panel controls.
Computer readout includes: A and B counts, the preset
value, and which counter is being displayed.
994
Dual Counter and Timer
3
