Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers - Manual (Page 45)

1 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 4-1
Choosing an addressing mode 4-3
Addressing block-transfer modules 4-7
Addressing summary 4-7
Assigning racks 4-8
Understanding PLC-5 controller memory 4-10
Addressing 4-16
Effectively Using I/O Memory 4-24
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.