32.1 INTRODUCTION
The basic purpose of an HMI is to allow easy graphical interface with a process.
These devices have been known by a number of names,
- Man Machine Interface (MMI)
- Human Machine Interface (HMI)
These allow an operator to use simple displays to determine machine condition and make simple settings.
The most common uses are,
- allow the operator to start and stop cycles
These devices allow certain advantages such as,
- color coding allows for easy identification (eg. red for trouble)
- pictures/icons allow fast recognition
- use of pictures eases problems of illeteracy
- screen can be changed to allow different levels of information and access
The general implementation steps are,
1. Layout screens on PC based software.
2. Download the screens to the HMI unit.
3. Connect the unit to a PLC.
4. Read and write to the HMI using PLC memory locations to get input and update screens.
To control the HMI from a PLC the user inputs set bits in the PLC memory, and other bits in the PLC memory can be set to turn on/off items on the HMI screen.
