20 Gear Drive, Plymouth Industrial Park, Terryville, CT 06786 page: 10
Tel: (860) 585-1254 Fax: (860) 584-1973 Web: www.amci.com
ControlLogix 7252 LDT Interface Manual
Revision 1.5
3. PWM Mode 1 (Variable Pulse with External Interrogation) – 7252 Default Mode
In PWM Mode 3, the signal interface is a pulse-width modulated RS-422 signal. In this mode the 7252 module is
configured for external interrogation and generates a start pulse to begin measurement. This start pulse will be a
minimum of 1.0 microsecond in duration. Within 50 nanoseconds after the leading edge of the start pulse has been
received by the transducer, the LDT shall generate an output pulse with variable width. The duration of this output
pulse is proportional to the distance from the null zone to the LDT magnet. The order of these pulses is illustrated in
the following figure.
4. PWM Mode 2 (Variable Pulse with Internal Interrogation)
This mode is similar to PWM Mode 1, except that no interrogation from the 7252 module is needed. Instead, the LDT,
when powered, will continually output pulse width modulated signals. The duration of this output pulse is proportional
to the distance from the null zone to the magnet. The order of these pulses is illustrated in the following figure.
Recirculation
The recirculation method is used to improve the resolution of systems using digital LDT when the “on” time of a pulse width
output is multiplied by a specific factor. This multiplication provides more counting time for the counter in the module’s
electronics, thus improving the resolution.
The 7252 module does not use recirculations because it utilizes high-speed counters with enough resolution to ensure
accurate measurements.
LDT sensors connected to the 7252 must be set for Number of Recirculations = 1, if this is a settable parameter.
