Programming; Human Machine Interfaces - Mitsubishi Electric FX Series Training Manual

General Introduction to PLCs
2.1.4

Programming

Ladder Logic
PLC's had to be maintainable by technicians and electrical personnel. To support this, the pro-
gramming language of Ladder Logic was developed. Ladder Logic is based on the relay and
contact symbols technicians were used to through wiring diagrams of electrical control panels.
The documentation for early PLC Programs was either non existent or very poor, just providing
simple addressing or basic comments, making large programs difficult to follow. This has been
greatly improved with the development of PLC Programming packages such as Mitsubishi's
Windows based, GX Developer.
Until recently there has been no formal programming standard for PLC's. The introduction of the
IEC 61131-3 Standard in 1998 provides a more formal approach to coding. Mitsubishi Electric
has developed a programming package, "GX IEC Developer" (Covered in detail later in this
document.). This enables IEC compliant coding to be adopted.
2.1.5

Human Machine Interfaces

The early programmable logic controllers interfaced with the operator in much the same way as
the relay control panel, via push-buttons and switches for control and lamps for indication.
The introduction of the Personal Computer (PC) in the 1980's allowed for the development of a
computer based interface to the operator, these where initially via simple Supervisory Control
And Data Acquisition (SCADA) systems and more recently via Dedicated Operator Control Pan-
els, known as Human Machine Interfaces (HMI). It is now common place to see PLC's heavily
integrated with these products to form user friendly control system solutions.
Mitsubishi offer a very wide range of HMI and SCADA products to suit a variety of operator Inter-
face applications.
2 - 2
It is now commonplace to find HMI's inte-
grated into PLC based control systems, pro-
viding the operator interface functionality.
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