I/o scanning – Rockwell Automation 1785-LTx,D17856.2.1 Classic PLC-5 Programmable Controllers Users Manual User Manual

Page 120

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Calculating Program Timing

Chapter 9

9-3

Effects of Different Input States on Logic Scan Time
You can write your logic so that it executes different rungs at different
times, based on input conditions. The different amounts of logic executed
in the logic scans causes differences in program scan times. For example,
the simple differences in rung execution in the following example cause
the logic scan times to vary.

00

JMP

rung 1

B3:0

20

I:000

02

MVM

MVM

JMP

O:013

20

LBL

02

rung 2

rung 3

rung 4

If I:000/02 is:

Rungs 2 and 3 are:

On

Skipped

Off

Executed

If you use subroutines, program scan times can vary by the scan time of
entire logic files.

I/O Scanning

The remote I/O scan cycle is the time that it takes for the processor
(configured as a scanner) to communicate with all of the entries in its rack
scan-list once. The remote I/O scan is independent of and asynchronous to
the program scan.

The scanner processor keeps a list of all of the devices connected to each
remote I/O link. An example system would look like this:

PLCĆ5/25

Rack 1

Rack 2

Rack 3

DH+

Rem I/O

I/O Status

Rack

Address

Rack

Size

1

2

3

Full

1/2

Full

In this example, the remote I/O channel continually scans the three racks in its scan list

and places the data in the remote I/O buffer in the processor. The processor updates

its own buffer and the I/O image table. During housekeeping, the two buffers are

updated by exchanging the input and output data with each other.

I/O Range

IO 010/00 to 017/17

IO 020/00 to 023/17

IO 030/00 to 037/17

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