Function Block Responses To Overflow Conditions; Timing Modes - Allen-Bradley Logix5000 Reference Manual

Function Block Responses
to Overflow Conditions
Response 1
Blocks execute their algorithm and check the
± ± ±
result for NAN or INF. If NAN or
± ±
block outputs NAN or INF.
ALMNTCH
DEDTPMUL
DERVPOSP
ESELRLIM
FGENRMPS
HPFSCRV
LDL2SEL
LDLGSNEG
LPFSRTP
MAVESSUM
MAXCTOT
MINCUPDN
MSTD
MUX

Timing Modes

Timing Mode Description
Periodic Periodic mode is the default mode and is suitable for most control applications. We recommend that you place the
instructions that use this mode in a routine that executes in a periodic task. The delta time (DeltaT) for the instruction is
determined as follows:
If the instruction
executes in a
Periodic task
Event or continuous
task
The update of the process input needs to be synchronized with the execution of the task or sampled 5-10 times faster than
the task executes in order to minimize the sampling error between the input and the instruction.
In general, the function block instructions that maintain history do not update
history with ±NAN, or ±INF values when an overflow occurs. Each instruction
has one of these responses to an overflow condition.
Response 2
Blocks with output limiting execute their
±
INF, the algorithm and check the result for
±
INF. The output limits are defined by the
HighLimit and LowLimit input parameters.
±
If INF, the block outputs a limited result.
±
If NAN, the output limits are not used and the
±
block outputs NAN.
HLL
INTG
PI
PIDE
SCL
SOC
These process control and drives instructions support different timing modes.
DEDT
DERV
HPF
INTG
LDL2
There are three different timing modes.
Then DeltaT equals
Period of the task
Elapsed time since the previous execution
The controller truncates the elapsed time to whole milliseconds (ms). For example, if the
elapsed time = 10.5 ms, the controller sets DeltaT = 10 ms.
Rockwell Automation Publication 1756-RM003N-EN-P - October 2011
Function Block Attributes
Response 3
The overflow condition does not apply. These
±
NAN or instructions typically have a boolean output.
BANDOSRI
BNOTRESD
BORRTOR
BXORSETD
CUTDTOFR
D2SDTONR
D3SD
DFF
JKFF
OSFI
LDLG
LPF
NTCH
PI
PIDE
Appendix B
RLIM
SCRV
SOC
TOT
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