Rockwell Automation 1785-LTx,D17856.2.1 Classic PLC-5 Programmable Controllers Users Manual - Manual (Page 84)

Selecting Interrupt Routines
Chapter 7
7-2
Program Execution States
User programs in the Classic PLC-5 processor are always in one of the
following five states: completed, ready, executing, waiting, or faulted.
Completed State
Program has completed execution
or has not yet started execution
Ready State
Program would be executing if it were of a higher priority;
all programs pass through this state; there can be
several programs in this state at any given time
Executing State
Program is executing; only
one program can be in this
state at one time
Waiting State
Program is ready for execution but is waiting
for some event to occur (such as an input to
transition or a timer to complete)
Completed State
Program has completed execution
or has not yet started execution
Has a new program
(e.g., an MCP, STI, PII)
with a higher priority
become ready?
Yes
No
Does the program fault?
Yes
No
Faulted State
A runĆtime error
has occurred within
the program
Does an appropriate fault routine
Yes
choose to clear the fault?
No
Program counter is
adjusted to point to
next instruction
All active user programs
are aborted and processor
enters faulted state
Does the program request Yes
No
a remote block transfer?
(STI and PII routines only)
While block transfer is taking place,
a rescheduling operation is performed
and lowerĆpriority programs are executed
Rescheduling Operation Rescheduling Operation Rescheduling Operation
(unless all other executions are prohibited by
a UID/UIE zone around the block transfer)
Waiting State