Bradley Smoker PLC-5 Example Calculation, Worst-case controller time is, Variable Value

C-12Maximizing System Performance

Example Calculation

Performance Effects of Online Operations

The worst-case controller time is:

Variable	Value
periodic input buffer update from remote I/O	10 ms
buffer
one program scan to guarantee inputs received	xx ms
one program scan to guarantee outputs received	xx ms
0.18 ms times number of racks	xx ms
total

For a 3-rack system with a 20 ms program scan, the worst-case controller time is: 10 + 20 + 20 + (0.18 ∗ 3) = 50.54 ms.

Based on the results of each throughput component calculation presented within the chapter, an example of a worst-case update time calculation is:

Variable	Value
input card delay	10 ms typical
I/O backplane	1 ms
worst-case remote I/O scan time	30 ms
worst-case controller time	50.54 ms
worst-case remote I/O scan time	30 ms
I/O backplane	1 ms
output card delay	1 ms typical
total	123.54 ms

The performance of the PLC-5 controller is affected when you perform online operations via a DH+ link to your program files while in Run mode. Affected activities are:

•DH+ messages

•serial port messages

•channel 3A messages

•remote block-transfers

Publication 1785-UM012D-EN-P - July 2005