Rockwell Automation 1772-LP3 PLC - 2/30 Programmable Controller Programming and Operations Manual - Manual (Page 43)

Data Table
Chapter 3
3Ć16
You program is a group of ladder diagram instructions used to control an
application. It is initially entered into memory using an industrial terminal.
Main Program
The main program follows the data table in memory and stores all the user
program instructions that make up the ladder diagram program. Most
instructions are stored in one memory word. Some advanced instructions
require up to 8 memory words. Unless specified elsewhere, instructions
require one word of user memory when the address is 377
8
or below and
two words of user memory when the address is 400
8
or above.
Assuming that the data table size has not been changed from
factory-configured values, the user program begins after word address
177
8
.
In certain applications, this area of memory can further be divided into data
highway instructions, main ladder diagram program and subroutine area.
Some of the simple program instructions, such as Examine On, use one
word of memory. Others, such as file instructions, are more complex and
can use two or more words of user program memory. As the user program
is entered from the industrial terminal, the number of words is indicated at
the right of the END statement (including data table words). The words
remaining in memory can be determined by subtracting that number from
the total memory available.
Subroutine Area
The Subroutine area contains instructions of special or often repeated
sections of program. Its upper boundary serves as the END of program
statement for the main program.
The Jump-to-Subroutine (JSR) instruction is an output instruction that
enables you to jump to a defined ladder diagram subroutine when desired.
You use subroutines to optimize program scantimes.
By pressing the SBR key on the -T3 industrial terminal, you define the
beginning of the Subroutine area. This may not be removed once inserted
except by clearing memory. The Subroutine area is not scanned unless a
JSR has been energized.
3.2.2
User Program