GATE A single LED indicates that the entire
instrument is enabled to count. For the Gate
LED to be illuminated, the module must be
placed in the Count mode (either manually or
via the interface option), the Enable input must
be above +3 V, and the preset stop condition
must not have been reached.
REMOTE A single LED labeled REM
indicates that the Model 994 is under
computer control, and all front-panel controls
are disabled. This mode is set by the
ENABLE_REMOTE command.
CONTROLS
DISPLAY Push-button selects the contents of
Counter A or B, or the Preset value for
presentation in the 8-decade display.
Repeatedly pushing the button cycles the
selection through the three choices as
indicated by the A, B, and Preset LEDs.
SELECT Push-button chooses the M, N, or P
digit in the display of the preset value. Pushing
the button advances the selection through the
three choices as indicated by the illuminated
LED. The Select push-button operates only if
the Preset mode has been selected by the
Display push button.
ADVANCE Push-button increments the preset
digit selected by the Select push-button once
each time the Advance button is depressed.
The M and N digit ranges are both 0 to 9. The
P digit range is from 0 to 6. The Advance
push-button operates only if the Preset mode
has been selected by the Display push-button.
TIME BASE Each push on this button
advances the selection one step through the
three time base choices of 0.01 Sec, 0.01 Min,
and Ext to determine the time base source for
the preset register.
STOP This push-button stops all sections of
the instrument from counting.
RESET Depressing this button resets both
counters to zero counts and turns Off both
overflow indicators. It also clears any counts
accumulated in the blind preset counter, but
does not change the selected preset value.
When power is turned On to the Module, a
Reset is automatically generated.
COUNT Pushing this button enables the
counting condition for the entire instrument,
providing the Enable input is not held below
+1.5 V and the preset value has not been
reached.
THRESH ADJUST (A and B) Front-panel
mounted, 25-turn trimpots to adjust the
positive input thresholds for Counters A and B.
The range is from +100 mV to +9.5 V. Adjacent
test points provide the TTL logic signal outputs
from the discriminators to facilitate adjustment
using an oscilloscope.
DWELL A one-turn potentiometer on the front
panel with an On/Off switch at the fully
counterclockwise position. Adjusts the display
dwell time over the nominal range from 1 to 10
seconds. When the instrument is in the
Recycle mode, dwell time occurs after the
preset value has been reached. Turning the
switch Off at the fully counterclockwise
position selects the Single Cycle mode. If the
print loop option is used, the Dwell control is
disabled when the print loop controller is active
and controlling the dwell time.
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. Zin = 1000 Ωto 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. Zin = 50 Ω; 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 nominally +5 V when
counting is enabled and <+0.5 V when
counting is disabled. Zo~30 Ω.
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 OVFL A
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
