Allen-Bradley POINT I/O 1734-VHSC24, User Manual, Page: 5 / 64

Publication 1734-UM003A-EN-P - August 2000
Preface 3
Definitions
The following define the intended operation of the Very High Speed
counter module.
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 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 not used, and not monitored).