PLC status bits for Allen-Bradley Control- Logix PLCs

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