Rockwell Automation 1785-LTx,D17856.2.1 Classic PLC-5 Programmable Controllers Users Manual - Manual (Page 121)

Calculating Program Timing
Chapter 9
9-4
I/O placed in the same chassis as the processor is called “processor-
resident” local I/O. These inputs and outputs are not updated during the
remote I/O scan—they are updated during the housekeeping portion of the
program scan. During housekeeping, the processor reads and writes the
I/O across the chassis backplane. Thus, the update of processor-resident
local I/O is synchronous to the program scan.
Figure 9.1 shows timing loops for discrete data transfer in a
PLC-5 processor.
Figure 9.1
Remote I/O Scan and Program Scan Timing Loops
Remote I/O
Buffer
x y
Adapter
Adapter
Adapter
Immediate I/O
IOT (x)
IIN (y)
ProcessorĆ
Rack
Data Exchange
I/O Image
Table
x
y
Resident
Data
Exchange
Rack 3
Rack 2
Rack 1
Remote I/O Scan Program Scan
Timing Loop Timing Loop
Read Inputs
Write Outputs
Housekeeping
Scan
Logic
During the housekeeping portion of the program scan, both the remote I/O
buffer and the processor-resident rack are updated. Remember that the I/O
scanner is constantly updating the remote I/O buffer asynchronously to the
program scan.