Human machine interfaces (HMI) are operator interface terminals with which users interact in order to control other devices. Some human machine interfaces include knobs, levers, and controls. Others provide programmable function keys or a full key pad. Devices that include a processor or interface to personal computers (PCs) are also available. Many human machine interfaces include alphanumeric or graphic displays. For ease of use, these displays are often backlit or use standard messages. When selecting human machine interfaces, important considerations include devices supported and devices controlled. Device dimensions, operating temperature, operating humidity, and vibration and shock ratings are other important factors.
Human Machine Interface Technologies
Many human machine interfaces include flat panel displays (FPDs) that use liquid crystal display (LCD) or gas plasma technologies. In LCDs, an electric current passes through a liquid crystal solution that is trapped between two sheets of polarizing material. The crystals align themselves so that light cannot pass, producing an image on the screen. LCDs can be monochrome or color. Color displays can use a passive matrix or an active matrix. Passive matrix displays contain a grid of horizontal and vertical wires with an LCD element at each intersection. In active matrix displays, each pixel has a transistor that is switched directly on or off, improving response times. Unlike LCDs, gas plasma displays consist of an array of pixels, each of which contains red, blue, and green subpixels. In the plasma state, gas reacts with the subpixels to display the appropriate color.
Performance and I/O Specifications
Human machine interfaces differ in terms of performance specifications and I/O ports. Performance specifications include:
Random access memory (RAM)
Hard drive capacity
Drive options (floppy drive or CD-ROM)
I/O interfaces allow connections to peripherals such as mice, keyboards, and modems. Common I/O interfaces include:
Ethernet: A local area network (LAN) protocol that uses a bus or star typology and supports data transfer rates of 10 Mbps
Fast Ethernet: Fast Ethernet is a 100 Mbps specification.
RS232, RS422, RS485: Balanced serial interfaces for the transmission of digital data
Small Computer System Interface (SCSI): Small Computer Systems Interface (SCSI) is an intelligent I/O parallel peripheral bus with a standard, device-independent protocol that allows many peripheral devices to be connected to the SCSI port.
Universal Serial Bus (USB). Universal serial bus (USB) is a 4-wire, 12-Mbps serial bus for low-to-medium speed peripheral device connections
Human machine interfaces are available with a variety of features. For example, some devices are web-enabled or networkable. Others include software drivers, a stylus, and support for a keyboard, mouse, and printer. Devices that provide real-time clock support use a special battery and are not connected to the power supply. Power-over-Ethernet (PoE) equipment eliminates the need for separate power supplies altogether. Human machine interfaces that offer shielding against electromagnetic interference (EMI) and radio frequency interference (RFI) are commonly available. Devices that are designed for harsh environments include enclosures that meet standards from the National Electronics Manufacturers' Association (NEMA).
The following are representative of the many standards that apply to Human Machine Interfaces.
ITU-T Z.351 Details Data Oriented Human-Machine Interface Specification Technique -- Introduction
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