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ControlLogix 7252 LDT Interface Manual
Revision 1.5
LDT Mode – Three Configuration Word bits are used to select the desired LDT mode, which can be one of the four
modes described in Chapter 2:
•
Start/Stop Mode (default)
•
Control Pulse Mode
•
PWM Mode 1
•
PWM Mode 2
LDT Gradient (Wire Speed) - This is a calibration parameter supplied by the LDT manufacturer and defines how fast
the pulse travels down the waveguide of the sensor. The data reported by the 7252 module will be repeatable, but not
accurate, if an incorrect Gradient value is used.
LDT manufacturers specify the gradient in µSec/inch, µsec/mm, or meters/second.
The gradient value used by the 7252 module has a range of 80000 to 99999, a default value of 90000 (for 9.0000
µsec/inch) and is always programmed in µsec/inch. Use the following formulas to convert from µsec/mm or
meters/second to µsec/inch.
Gradient in µsec/inch = (Gradient in µsec/mm * 25.4 mm/inch)
Gradient in µsec/inch = 1 / (Gradient in meters/sec * 0.00003937)
Full Scale Length - Most commonly set to the expected length of travel, the Full Scale Length can
actually be any value that simplifies the Full Scale Count value. Based on the Measurement Unit
setting, the range of values is 2 to 800 inches or 50 to 20320 millimeters. Default value of 16 inches.
Full Scale Count - Sets the number of counts over the specified Full Scale Length. The range of values
is 2 to (Full Scale Length * 1000) if Measurement Unit is inches or 2 to (Full Scale Length * 10) if
Measurement Unit is millimeters. Default value of 16000. (Resolution equals 0.001 inches)
Preset Value: The zero position of the LDT encoder’s Data Value may not match the zero position of the application
machine. The Preset Value parameter gives you the ability to offset the Data Value from the actual LDT data to a
value that will be more useful for your application.
Programming the Preset Value parameter does not change the Data Value. It is stored in the 7252 module’s
memory until the module sees a zero to one transition of the Apply Preset bit in the output registers.
Apply Preset: Offsetting the Data Value to the Preset Value is a two step operation. First, a Message Instruction must
be used to send the Preset Value with the other setup parameters to the 7252 module. Second, setting the Apply Preset
bit in the output registers will change the Data Value to the Preset Value.
Setting the Apply Preset bit causes the module to generate an internal offset value that is applied to the Data
Value before it is reported to the PLC. This internal offset is saved in the 7252 module’s Flash memory, so it is
not necessary to home the module at every power up. Please note that using a Message Instruction to program a
channel’s setup data will clear the internal offset generated by an Apply Preset operation.
The 7252 module’s Flash memory is guaranteed for 10,000 write cycles before
writing to it will cause it to fault. Therefore continuously applying the Preset should
be avoided. If your application requires you to continuously apply the Preset,
consider calculating and applying the Preset in your PLC program.
