Plc Operation; Introduction; Plc Operating System - AutomationDirect C0-USER-M Manual

PLC Operation

Introduction

Achieving proper control of your equipment or process requires a thorough understanding of
how the CLICK PLC controls all aspects of system operation. There are three main areas to
understand before you create your application program:
• PLC Operating System – the PLC manages all aspects of system control. A quick overview
of all the steps are provided in the next section.
• PLC Operating Modes – The two primary modes of operation are Stop mode and Run
mode.
• PLC Memory Map – CLICK PLCs offer a wide variety of resources, such as timers,
counters, inputs, etc. The Memory Map section shows the organization and availability
of these data types.

PLC Operating System

At powerup, the CLICK PLC initializes the internal
electronic hardware. Memory initialization starts with
examining the retentive memory settings. In general, the
contents of retentive memory are preserved, and non-
retentive memory is initialized to zero (unless otherwise
specified).
After the one-time powerup tasks, the PLC begins the
cyclical scan activity. The flowchart to the right shows
how the tasks differ, based on the PLC mode and the
existence of any errors. The "scan time" is defined as the
average time around the task loop. Note that the PLC is
always reading the inputs, even during Stop mode. This
allows programming tools to monitor input status at any
time.
The outputs are only updated in Run mode. In Stop
mode, they are in the off state.
Error detection has two levels. Non-fatal errors are
reported, but the PLC remains in its current mode. If a
fatal error occurs, the PLC is forced into Stop mode and
the outputs turn off.
CLICK PLC Hardware User Manual, 6th Edition, Rev. F – C0-USER-M
Chapter 2: Specifications
Power up
Initialize hardware
Initialize various memory
based on retentive
configuration
Update input
Service peripheral
Update Special Relays
STOP
Mode?
RUN
Execute program
Update output
Do diagnostics
YES
OK?
NO
Report error , set flag
register , turn on LED
NO
Fatal error
YES
Force CPU into
STOP mode
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