![background image](http://html1.mh-extra.com/html/allen-bradley/plc-2-30/plc-2-30_programming-and-operations-manual_2900216030.webp)
Data Table
Chapter 3
32
A group of 16 bits makes up a word. This word can be thought of as being
made up of two 8-bit bytes; a lower byte and an upper byte.
Because of its function in memory, one PLC-2/30 word may also be
thought of as a memory location: when a word is being used, an actual
physical location in memory is being accessed.
A specific bit in memory can be identified by combining the word address
and bit number to form the bit address, such as 030/12 or 1701/04. The bit
address is shown by writing the word address above the instruction and the
bit number below it.
The processor can have a memory capacity of up to 16,256 words. These
memory words are organized by their word address and are divided into
three major areas (Figure 3.2):
Data table
User program
- Main Program
- Subroutine Area
Message Storage Area
All input/output status and user program instructions are stored in one of
these parts (Figure 3.2).
Data table words, and/or the 16 bits in each word, are controlled and
utilized directly by the processor. The processor uses the status of input
devices and the control logic established in the user program to determine
the status of output devices. Transfer of input data from input devices and
transfer of output data to output devices occurs during the I/O scan. If
the output instruction’s status changed in the program, the actual output
device’s on/off status is updated during the I/O scan to reflect this change.
3.2
Memory Organization
3.2.1
Data Table
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com