GE Fanuc Programable Logic Controller

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The Series 90 Micro powered by 12 volt power supply.

Background:

Late in 1998 Motherwell Bridge contacted the Science Teachers' Association of Western Australia (STAWA) and the Educational Computing Association of Western Australia (ECAWA) with an offer to schools of free Series90 Micro PLCs from Ge Fanuc.

Around eight schools took up the offer and have established 'sets' of PLCs for use in various Learning Areas.

Motherwell Bridge is continuing to provide technical support for schools involved.

This site aims to provide basic support for schools using the Series 90 Micros. It makes no claims to represent any of the professional associations, or corporations mentioned.

The Series 90 Micro PLC has eight input lines and six output lines. It may be powered from 12 volt, or 24 volt power supplies. Once programmed it will hold the commands for many months in "Flash Memory" even though the power may be disconnected. The input lines are 'digital' lines able to read a simple ON/OFF voltage state. (It is possible to modify the PLC to cater for analogue input.) Each of the output lines controls a relay. Any device connected to an output must have its own separate power supply.

The internal bus lines of the Micro 90 PLC are 'opto-isolated' from the input switching circuit. Each input line has the equivalent of a Light Emitting Diode (LED) and a photo-transistor as a receiver. This arrangement protects the internal circuitry against over-voltage input.

The Micro 90 has eight input lines. The typical input circuit consists of a 24 volt supply with a switch. One end of the circuit connects to a "COMMON" terminal and the other end connects to one of the eight input lines. The supply may be as indicated below, or reversed, with the negative line connected to the COMMON. (Swapping the polarity changes from positive to negative 'logic'.)

The Micro 90 has a built-in 'voltage-doubler' that converts the 12 volt supply to 24 volts output. This 24 volt source can be used as part of the input circuits, avoiding the need for a separate power supply. The arrangement of the circuit becomes a little more confusing. Keep in mind that it is basically just like the one above.

The Micro 90 has six output lines. Each output line from the CPU controls a RELAY.

A relay consists of a coil of wire acting as an 'electro-magnet'. When current flows through the coil it develops a magnetic field that attracts a pivoted 'arm'. The arm has a return spring attached to return it to its original position when the magnetic field is turned off. At the end of the 'arm' is an electrical 'contact'. When the relay switches 'ON', the arm moves down and touches the contact. The circuit from the "COMMON" to the 'Output Line' is completed. ie the relay switches ON.

A relay is therefore simply an ON/OFF switch. In this example the relay is controlled by the CPU inside the PLC. Whatever LOW VOLTAGE (ie up to 32 volts) device you could control with a switch can be controlled by the PLC.

Remember, the output lines are simply switches. You must provide the power supply needed to drive any devices you connect to an output line.

In the example above, the globe will light when the pushbutton is pressed (assuming you have programmed the PLC appropriately).