Bradley Smoker PLC-5 user manual Addressing I/O and Controller Memory

Chapter 4

Addressing I/O and Controller Memory

Using This Chapter

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

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:

Classification	Term	Relation to Controller Memory
A specific terminal on an I/O module that	terminal or	The density of an I/O module, i.e., 8-point, 16-point, 32-point, directly
occupies a space in controller memory	point	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	I/O group	16 input bits = 1 word in controller’s input image table
word in controller’s input image table and 1		16 output bits = 1 word in the controller’s output image table
word in the controller’s output image table.

Controller memory needs to be grouped so that I/O rack related I/O groups can be considered a unit.

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.

Publication 1785-UM012D-EN-P - July 2005