Rockwell Automation 1734-IK Encoder/Counter Modules User Manual - Manual (Page 9)

Publication 1734-UM006B-EN-P - August 2005
Preface 3
Definitions The following define the intended operation of the Encoder/Counter
module.
Term Definition
Lead Breakage Typically requires a shunt resistor (across the load) to detect
3 levels of current/input states -
• Open (Wire Off, Device = ?)
• Off (Wire OK, Device Off)
• On (Wire OK, Device On)
This method does not check the input against a time base,
only that the device wiring (current loop) is intact
Missing Pulse Typically uses an input pulse to reset a watchdog timer (fixed
or programmable HW). This method does detect “Lead
Breakage”, since a broken wire will time-out the watchdog.
Zero Frequency Typically uses an input pulse to calculate an input frequency
and verify it is above an error threshold. This method does
detect Lead Breakage, since a broken wire will generate a
0Hz frequency.
Missing Pulse or Zero Frequency will also detect a customer
device stuck high or low, since the counter is monitoring for a
change in the input state. Currently, the Counter/Encoder
Modes do not have Zero Frequency Detection - the A & B
inputs are time independent, only looking for input edge
changes to increment/decrement the count value.
The Period/Rate and Continuous Rate modes do have Zero
Frequency Detection, since the Z input is monitored for Zero
Frequency in Firmware (A and B inputs are not used and not
monitored).
The Rate Measurement mode inherently has Zero Frequency
Detection, since no A pulses in any sample period are = 0Hz
(B and Z inputs are not used and not monitored).