AMETEK 955D LDT - Manual (Page 5)

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3.1 CP (Control Pulse)
The Control Pulse signal interface of the BRIK digital
output series is a differential RS-422 output. The maxi-
mum cable length for the differential digital LDT’s is 1,500
feet. To initiate a start pulse, an external device is used.
This start pulse should be 1.0 microsecond in duration.
After the start pulse is received, the LDT will generate
a stop pulse of 1.0 microsecond in duration. The time
between the leading edge of the start pulse to the lead-
ing edge of the stop pulse is proportional to the distance
from the Null Zone to the Magnet. The order of these two
pulses is illustrated in the Figure 3-1. To wire the 955D-
CP, see Figure 3-6. For proper grounding information,
see Section 3.4.
3.2 VP (Variable Pulse)
The Variable Pulse signal interface of the BRIK digital
output series is a pulse width modulated signal (RS-422).
The maximum cable length for the differential LDT’s is
1,500 feet. This LDT can also be confi gured for external
or internal interrogation. External interrogation is when
an external device connected to the LDT generates a
start pulse. This start pulse should be a minimum of 1.0
microsecond in duration. Within 50 nanoseconds after
the leading edge of the start pulse has been received,
the LDT will generate an output pulse. The duration of
the output pulse is proportional to the distance from
the Null Zone to the Magnet . The order of these two
pulses is illustrated in Figure 3-2. The 955D-VP can also
generate internal interrogations. This LDT will continually
output pulse width modulated signals. As with a 955D-
VP using an external interrogation, the duration of this
output pulse is proportional to the distance from the Null
Zone to the Magnet. This is illustrated in Figure 3-3. To
wire the 955D-VP, see Figure 3-6. For proper grounding
information, see Section 3.4.
Recirculations
The method used to improve the resolution of a system
using a digital LDT. The “on” Time of a pulse width
output is multiplied by a specifi c factor (from 1-127).
This multiplication provides more counting time for the
counter in the customer’s electronics, thus improving
the resolution. The only disadvantage to the higher
recirculation numbers is the time needed to process the
signal.
Chapter 3 955D Outputs
Figure 3-1 955D-CP
1 MICROSECOND (RECOMMENDED)
0.2 MICROSECONDS (MINIMUM)
+ INPUT (START PULSE)
TIME BETWEEN PULSES IS PROPORTIONAL TO
DISTANCE BETWEEN MAGNET AND HEX HEAD
+ OUTPUT (STOP PULSE)
TIME BETWEEN PULSES IS PROPORTIONAL TO
DISTANCE BETWEEN MAGNET AND NULL ZONE
Figure 3-2 955D-VP with External Interrogation
INPUT (INTERROGATION)
1 MICROSECOND (RECOMMENDED)
0.2 MICROSECONDS (MINIMUM)
WIDTH OF PULSE IS PROPORTIONAL TO
DISTANCE BETWEEN MAGNET AND HEX HEAD
OUTPUT PULSE
WITHIN 50 NANOSECONDS AFTER INTERROGATION
TO DISTANCE BETWEEN MAGNET AND HEX HEAD
WIDTH OF PULSE IS PROPORTIONAL TO
DISTANCE BETWEEN MAGNET AND NULL ZONE
WITHIN 50 NANOSECONDS AFTER INTERROGATION
PULSE WIDTH MODULATED OUTPUT SIGNAL BEGINS
Figure 3-3 955D-VP with Internal Interrogation
OUTPUT PULSE
LOW OUTPUT VARIES DEPENDING ON LENGTH
PROGRAMMED VIA DIP SW2 SWITCHES 1-6
WIDTH OF PULSE IS PROPORTIONAL
TO DISTANCE BETWEEN MAGNET AND HEX HEAD
WIDTH OF PULSE IS PROPORTIONAL TO
DISTANCE BETWEEN MAGNET AND NULL ZONE
LOW OUTPUT VARIES DEPENDING ON THE
INTERROGATION TIME AND THE POSITION
OF THE MAGNET