Programming Logic Controllers

Selection, Components, & Communication - Chapter 2

PLC

• Process control computers• no number crunching no graphics

• Event driven industrial control systems

• Environmentally hardened• Dust, temp, noise (em)

• Easy• Repair, operate, and program

PLC Selection

• Mfg support• Serviceability• Flexibility• Expandability• Programming software• Training• Documentation

Allen-Bradley SLC 500

• Fixed• Single housing for power supply, cpu, I/O

modules

• Modular

Allen-Bradley SLC 500 Components

• Chassis• Rack / backplane for PLC and modules• Distributes power• slotted

• Power Supply – sufficient for all components• Processor – chosen for memory and math • I/O Modules

• Digital I/O modules• Analog I/O modules• Specialty• Distributed / networking I/O modules

Addressing• Slot 0 next to power supply

• CPU

• Slot 1+ hold other modules• I:1 (input module in slot 1)• O:2 (output module in slot 2)• I:3, O:4, I:5, O:5 May have many modules

• Each module has several ports/terminals• I:1/2 input on module 1, terminal 2• O:2/1 output on module 1, terminal 1

• A Fixed PLC may have both Input and output in slot 0• I:0/0• O:0/0

Sourcing and Sinking

• Sourcing and Sinking refer to electrical current flow

• Sourcing modules connect to sinking modules and vice versa

• Sourcing occurs when current flows out of PLC circuit

• Sinking occurs when current flow into PLC port

PLC Memory Expansion and Usage

System memory (25%) User Memory (75%)User Memory• Ladder logic program(s)System Memory• Input status• Output status• Timers’ status• Counters’ status• Math and logic instruction status

Communications

• Channel 1 and Channel 0 occasionally• Channel 0 for peripheral devices• Bar code scanner• Printer

• Channel 1• Communication with other plc/pcs

• RS232 interface for programming• Ethernet or other network communications

Communications

• Baud Rate• Parity• DTE (initiate) and DCE (respond)• Simplex, Half Duplex, Full Duplex

Programming Modules

• Hand held• Common for small devices

• Dedicated• Part of the PLC circuit/station

• Computer• Need a device to translate• States/setting• Off/program/disabled/run/forced