Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers - Manual (Page 70)

Publication 1785-UM012D-EN-P - July 2005
5-2 Communicating with Controller-Resident I/O
Program Scanning 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
b
read inputs
Controller-
Rack
Resident
I/O Image
Table
Data
Exchange
Housekeeping
Logic
Scan
Program Scan
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
Update
I/O image
Extended-
local I/O
Remote I/O
Buffer
Data
Exchange
I/O Scan
Data
Exchange
a
b
a
write outputs
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.
Logic Scan
Housekeeping