Rockwell Automation 1772-LP3 PLC - 2/30 Programmable Controller Programming and Operations Manual - Manual (Page 145)

Output Override and I/O Update Instructions
Chapter 7
7Ć3
WARNING: MCR or ZCL zones should not be overlapped or
nested. Each zone should be separate and complete.
Overlapping MCR or ZCL zones may result in unpredictable or
hazardous machine operation with possible damage to
equipment or personal injury.
Two instructions used to update I/O data during the execution of the user
program are:
Immediate Input –|I|–
Immediate Output –(IOT)–
These instructions are used to transfer critical I/O data ahead of the normal
scan sequence. This speeds up the response of output devices to the
program and the update of input data for program use.
The immediate I/O instructions are usually used where I/O modules
interface with I/O devices that operate in a shorter period than the
processor scan time. These may include TTL logic or fast response input or
output devices.
Most electromechanical devices have a response time longer than the
processor scan time. Thus, data to and from these devices need not be
updated ahead of the normal I/O scan.
The PLC-2/30 processor scan sequence can be divided into 2 parts
(Figure 7.2):
Program Scan
I/O Scan
7.2
I/O Updates
7.2.1
Scan Sequence