The man–machine interface

Abstract

This chapter discusses man-machine interface. PLC has to “connect” to human operators, accepting commands from them and displaying the status of the plant in a form that can readily and easily be understood. A digital display driven in BCD normally uses one of the unused binary codes, 1010 to 1111, to generate a blank display. Simple digital inputs (pushbuttons, joysticks, switches) are normally allocated one input per motion and do not require anything particularly special. Concerning PLC with analog signals, flows, temperatures, pressures, etc., these will usually be displayed to the operator. A graphics terminal has just two connections to the outside world, a serial link connection and a power supply. The Allen Bradley Panelview family was designed to replace switches, pushbuttons, numeric displays, and similar devices. The Panelview built up a graphics screen with semigraphic characters placed on a screen with a “pick and place” menu approach. Siemens provides a vast range of operator terminals from simple text-only message displays to complete computer-based Scada systems. One major advantage of graphical displays is that mistakes can be rectified without leaving scars, albeit at some cost. The operator will need to input data and initiate actions. Where a simple text message is to be displayed, possibly with embedded data, message displays driven by a simple serial link carrying ASCII coded characters can be used. PCs have much better graphics capability than the average PLC, so it is not surprising that PCs are increasingly being used as the link between humans and PLC systems.

Document type: Part of book or chapter of book

Original document

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DOIS: 10.1159/000393828 10.1016/0040-1625(86)90059-4 10.1088/0305-4624/10/3/i04 10.1016/b978-0-7506-0498-7.50010-4