Allen-Bradley LOGIX 5000 Reference Manual page 95

Process Control Instructions Chapter 1
The high-high to low-low alarming algorithm compares the deviation to deviation
alarm limits and the deviation alarm limits plus or minus the deadband.
(1) During instruction first scan, the instruction clears the deviation alarm
outputs. The instruction also clears the deviation alarm outputs and disables the
alarming algorithm when PVFaulted or PVSpanInv is set.
Zero Crossing Deadband Control
You can limit CV such that its value does not change when error remains within
the range specified by ZCDeadband (| E | ≤ ZCDeadband).
(1) When ZCOff is cleared, ZCDeadband > 0, error has crossed zero for the first
time, (i.e. En >= 0 and En-1 < 0 or when En <= 0 and En-1 > 0), and | En | <=
ZCDeadband, the instruction sets ZCDeadbandOn.
Rockwell Automation Publication 1756-RM006K-EN-P - November 2018 95