Bradley Smoker PLC-5 user manual Program Scanning, 2Communicating with Controller-Resident I/O

5-2Communicating with Controller-Resident I/O

The controller performs two primary operations: program scanning where

-logic is executed

-housekeeping is performed

I/O scanning - where input data is read and output levels are set

During logic scan, inputs are read from and outputs are written to the I/O image table.

During housekeeping, data exchange occurs between the I/O image table and the remote I/O buffer, extended local I/O, and controller-resident rack.

Data

Exchange

Remote I/O

Buffer

a

b

I/O Scan

Extended- local I/O

		Data
	Exchange
						Housekeeping
I/O Image			Update
Table
			I/O image
		Data
	Exchange
Controller-
Resident
					Logic
Rack
				Scan
		a
		write outputs
		b				Program Scan
		read inputs

Program Scanning

Logic Scan

Housekeeping

The program scan is the time it takes the controller to execute the logic program once, perform housekeeping tasks, and then start executing logic again.

The controller continually performs a logic program scan and housekeeping. Housekeeping activities for PLC-5 controllers include:

•performing controller internal checks

•updating the input image table with:

–controller-resident input module data

–remote input module data as contained in the remote I/O buffer

–extended local I/O input module data

•sending output image table data to:

–controller-resident output modules

–remote I/O buffer

–extended local I/O output modules

Publication 1785-UM012D-EN-P - July 2005