
Introduction to Programming
Chapter 4
412
This procedure allows the processor to make a smooth transition from
one form of the rung to the other form during the time the branch start
instruction is being completed.
The PLC-2/30 controller does not require that an END (of program)
statement be entered by the user after the last program instruction. An
END statement is generated by the processor. It is present before any
program steps are entered and is automatically positioned after the last
programmed instruction, once program entry is begun. An additional
temporary END statement can be entered, although its use is not
mandatory.
Starting with the first program instruction, the processor scans the program
and executes all instructions in the sequence called for by the program. The
END statement stops the processor from scanning unused memory. Inputs
and outputs are then scanned. The processor returns to the first program
instruction and begins another program scan.
In normal operation, an END statement is displayed on the industrial
terminal when the cursor is moved past the last user program instruction.
The END statement also appears before program steps are entered. When
a user-supplied teletypewriter or keyboard/printer is used, the END
statement is printed on the hardcopy printout. At the right of the END
statement, a 3- or 4-digit number appears. These digits indicate the number
of decimal words actually entered into memory before and including the
END statement.
NOTE: The size of the data table must be subtracted from the number
displayed in order to determine the exact number of user program words.
Should a user attempt to enter more instructions than the maximum
capacity of the available memory, a MEMORY FULL message is
displayed on the industrial terminal screen. Additional program
instructions cannot be entered.
Most PLC-2/30 instructions take an average of 3 to 6 msec for the
processor to scan and execute. The execution time for different instructions
varies considerably and is dependent on the exact instruction and its
true/false state. A typical program using 1,024 words of memory
(including the data table) would have an execution time of approximately
5 msec, assuming a distribution with approximately 80% relay-type
instructions, such as Examine On, Examine Off, Output Energize.
4.3.4
Ending a Program
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