PLC Basic

PLC Basic

Introduction

This guide will describe the terminology needed for basic operation of the SYSMAC PLC (Programmable Controller), for clients new to our PLCs.
This guide does not cover technical details regarding the setting of the FA System or PLC programming after purchase.

I/O Refresh

With the PLC (Programmable Controller), commands from user programs designed by the client are executed one by one and processed by reading and writing information in the internal PLC memory area (At OMRON, we call this the "I/O Memory").
At the same time, packages of data from sensors/switches that are directly connected to the basic I/O unit are exchanged with data in the PLC internal I/O Memory, at specific times. This process to totally exchange external data and internal I/O Memory data is called "I/O Refresh Operation".
It is important to know the timing by which the I/O Refresh will be executed when considering the operation of the FA System and User Programs designed by the client. In the case of the SYSMAC PLC, this I/O Refresh operation is performed immediately following the execution of all other commands. (See Figure below)

Cycle Time

In terms of the PLC processing cycle, the cycle time is the time from the execution (commencement) of the I/O Refresh operation to the execution (processing) of the following I/O Refresh.
The cycle time includes time for overhead processing (self-diagnosis), execution of user programs, I/O Refresh processing and the processing of peripheral services.

When the cycle time is long, the cycle for updating data from outside of the PLC and the I/O response time are also longer, making it impossible to implement changes that are input at a rate faster than the cycle time.

When the cycle time is short, I/O response time is also shortened, which allows high speed processing.

As the cycle time changes, the command execution cycle and I/O response times also change.

In the case of the SYSMAC PLC, the cycle time can be requested in the following manner:
cycle time = Overhead Processing Time + Total Command Execution Time + I/O Refresh Time + Peripheral Service Time

The calculation methods for each execution time for the SYSMAC PLC are included in the product manual.

Interrupt Tasks

Normally, user programs are executed in order along with the processing of the I/O Refresh etc., within the PLC processing cycle (See "I/O Refresh").
Interrupt Tasks however are executed in precedence to this processing cycle. In the event that certain interrupt conditions are met, the processing cycle will be suspended and the interrupt tasks will be executed first.
(The SYSMAC PLC sometimes refers to the "Interrupt Tasks" as "Interrupt Programs", but here we shall use "Interrupt Tasks", the terminology used in the CS/CJ Series manuals)
For example, in the case of the CS/CJ Series, Interrupt Tasks can include power Off interrupt, Scheduled Interrupts, I/O Interrupts, Periodic Interrupts based on the internal timer, and External Interrupts.

Major Interrupt Tasks	Details
Power Break Interrupt Tasks	Executed during a power break.
Scheduled Interrupt Tasks	Executed based on a specific schedule.
I/O Interrupt Tasks	Executed at the start-up of a connected Interrupt Input Unit.
External Interrupt Tasks	Executed when requests are received from the Special I/O unit, the CPU Bus Unit, and the INNER Board (only for CS Series).

I/O Allocation

In order for user programs to utilize I/O signals from the I/O Units mounted to the PLC, it is necessary to first assign an address within the PLC I/O Memory.
The assignment of I/O Memory to the input or output from these units within the PLC is known as I/O Allocation. The CPU Unit uses this I/O Allocation information in the operation of I/O Refresh with connected units.
In the case of SYSMAC PLC, this I/O Allocation information is recorded in the PLC in the "Registered I/O Table". This "Registered I/O Table" can be created by either automatically registering online with programming tools utilizing information from the unit built-in to the PLC, or by using programming tools to design offline and then automatically registering by transmitting the I/O Table to the PLC. (Some devices may not require the creation of a Registered I/O Table, and others may not support the offline design of I/O Tables.)

Online Automatic Registration

Offline Automatic Registration

CPU Unit Memory Area

The PLC Unit utilizes a variety of different data including user programs, I/O Memory data and comments, CPU Unit and Special I/O units Parameters, and Registered I/O Table information etc.
All of this data that is used by the PLC is stored in the memory area within the CPU Unit.
The SYSMAC PLC has the following types of Memory Areas, which is backed up by a battery.
In the case of the SYSMAC CS/CJ Series, the content of the Memory Area is backed up using flash memory, which means that even if battery power fails, any user programs and parameter area data will not be lost.

User Program Area

This records user programs designed by the client.

I/O Memory Area

This Area is accessed by command operands. It records information such as the CIO, Internal I/O Area, holding area, auxiliary area, DM Area, EM Area, Timer Completion Flags/Present Value, Completion Flag/Present Value, Task Flags, Index Register, Data Register, Condition Flags, Clock Pulse, etc. The data in the I/O Memory Area locates in or is in areas in which the contents are cleared every time the power is turned back on, and areas in which prior information is retained. (In some areas you can select whether to clear or retain.)

Parameter Area

This contains all of the information regarding initial parameters used by the PLC.
It records information such as the PLC System Parameters, Registered I/O Table, Routing Table, and PLC Setup for CPU Bus unit