Chapter 4 - addressing i/o and controller memory, Addressing i/o and controller memory, Using this chapter – Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers User Manual
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Publication 1785-UM012D-EN-P - July 2005
Chapter
4
Addressing I/O and Controller Memory
Using This Chapter
I/O Addressing Concept
Since the main purpose of a programmable controller is to control inputs and
outputs of field devices like switches, valves, and thermocouples, these inputs
and outputs must occupy a location in the controller memory so that they can
be addressed in your control program. Each terminal on an input or output
module that can be wired to a field device occupies a bit within controller
memory. The part of controller memory that houses I/O addresses is the
input image table and the output image table.
I/O addressing helps connect the physical location of an I/O module terminal
to a bit location in the controller memory. I/O addressing is just a way to
segment controller memory. The segmentation is as follows:
For Information About
Go to Page
I/O addressing concept
Choosing an addressing mode
Addressing block-transfer modules
Addressing summary
Assigning racks
Understanding PLC-5 controller memory
Addressing
Effectively Using I/O Memory
Classification
Term
Relation to Controller Memory
A specific terminal on an I/O module that
occupies a space in controller memory
terminal or
point
The density of an I/O module, i.e., 8-point, 16-point, 32-point, directly
relates to the amount of memory (bits) the module occupies in controller
memory. For example, a 16-point input module occupies 16 bits in the
controller’s input image table.
I/O terminals that when combined occupy 1
word in controller’s input image table and 1
word in the controller’s output image table.
I/O group
16 input bits = 1 word in controller’s input image table
16 output bits = 1 word in the controller’s output image table
Controller memory needs to be grouped so that
related I/O groups can be considered a unit.
I/O rack
128 input bits and 128 output bits, or8 input words and 8 output words,
or 8 I/O groups
Each PLC-5 controller has a finite amount of racks it can support. For
example, a PLC-5/30 can support 8 I/O racks.
The controller always occupies at least one I/O rack for itself, rack 0 by
default.