Rockwell Automation 1785-LTx,D17856.2.1 Classic PLC-5 Programmable Controllers Users Manual - Manual (Page 138)

Chapter 10
Maximizing System Performance
10-6
The processor time is the time needed to process the inputs and set the
corresponding outputs. This processor time varies for different processors
and is based on input buffering, program scan, etc.
In a PLC-5 system, both inputs and outputs are buffered between the I/O
image table and the I/O scanner. The movement of inputs from the scanner
to the input buffer is asynchronous to the movement of data from the input
buffer to the input image table. The worst-case processor time is:
Periodic input buffer update
One program scan to guarantee
inputs received
.18 ms times the number of racks
= 10 ms
= xx ms
= xx ms
One program scan to guarantee
outputs received = xx ms
For a 3-rack system with a 20 ms program scan, the worst-case processor
time is: 10 + 20 + 20 + .54 = 50.54 ms.
To calculate throughput, use the following equation:
Input
Card
Delay
I/O Backplane
WorstĆCase
Remote I/O
Scan Time
WorstĆCase
Processor
Time
+ + +
WorstĆCase
Remote I/O
Scan Time
+ I/O Backplane +
Output
Card
Delay
+
An example of a worst-case update time calculation:
Input card delay
I/O backplane
WorstĆcase remote I/O scan time
WorstĆcase processor time
WorstĆcase remote I/O scan time
I/O backplane
Output card delay
= 20 ms (typical)
= 1 ms
= 30 ms
= 50.54 ms
= 30 ms
= 1 ms
= 8.8 ms (typical)
Total 141.34 ms
Processor Time
Calculating Throughput