What is a Status register?
A status register is a hardware register which contains the information about the status of the processor. It is a collection of flags bits in a Programmable status word. Status memory allows a program to check the PLC operation, and also make some changes.
Here we discuss the PLC status bits of Allen-Bradley Control-Logix PLCs. More complete lists are available in the manuals. The first six bits are commonly used and are given simple designations for use with simple ladder logic. More advanced instructions require the use of Get System Value (GSV) and Set System Value (SSV) functions.
Immediately accessible status values:
S:FS - First Scan Flag S:N - The last calculation resulted in a negative value S:Z - The last calculation resulted in a zero S:V - The last calculation resulted in an overflow S:C - The last calculation resulted in a carry S:MINOR - A minor (non-critical/recoverable) error has occurred
A selected list of status bits is shown below for Allen-Bradley Micrologic
S2:0/0 carry in math operation
S2:0/1 overflow in math operation
S2:0/2 zero in math operation
S2:0/3 sign in math operation
S2:1/15 first scan of program file
S2:8 the scan time (ms)
S2:18 year
S2:19 month
S2:20 day
S2:21 hour
S2:22 minute
S2:23 second
S2:28 watchdog setpoint
S2:29 fault routine file number
S2:30 STI (selectable timed interrupt) setpoint
S2:31 STI file number
S2:46-S2:54,S2:55-S2:56 PII (Programmable Input Interrupt) settings
S2:55 STI last scan time (ms)
S2:77 communication scan time (ms)
Examples of SOME values available using the GSV and SSV functions:
CONTROLLERDEVICE - information about the PLC
PROGRAM - information about the program running
LastScanTime MaxScanTime TASK EnableTimeout LastScanTime MaxScanTime Priority StartTime Watchdog
WALLCLOCKTIME - the current time
DateTime DINT[0] - year DINT[1] - month 1=january DINT[2] - day 1 to 31 DINT[3] - hour 0 to 24 DINT[4] - minute 0 to 59 DINT[5] - second 0 to 59 DINT[6] - microseconds 0 to 999,999
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