Chapter 14 Understanding Relay Instructions and the Programmable Controller Input Modules

Objectives (1 of 2) Explain proper programming of normal closed and normally open PLC input signals. Describe hardware relay operation and its correlation to proper PLC interface and programming. Convert a conventional start-stop schematic to a PLC ladder rung diagram.

Objectives (2 of 2) Explain why PLC ladder logic for motor starter interface has an additional contact in comparison to a conventional schematic. Hook up a start-stop station to a PLC input module. Hook up and develop a PLC program to correctly control a motor starter.

Typical Hard-Wired Start-Stop Latching Circuit

Incorrect Conversion of Conventional Start-Stop Schematic to PLC Control

Normally Open Pushbutton and Non-energized Relay

Normally Open Pushbutton Energizing Relay Coil and Pilot Light B

Normally Closed Pushbutton Energizing Relay Coil and Pilot Light B

Normally Closed Stop Pushbutton Depressed

Relay Status in Conjunction with Input Pushbuttons

Input and Output Separation

Separated Inputs Connected to PLC Input Screw Terminals

Start Push-Button Status Reflected in the Input Status Table

Start-Stop Input Bit Status from Input Hardware in a Non-energized State

Normally Open Pushbutton Not Activated

Normally Open Pushbutton Input Pressed

Normally Closed Input Pushbutton

Normally Closed Pushbutton Input to PLC

Pushbutton Released Output Latched through CR 1-1

Conventional Schematic Start-Stop Logic

Conventional Motor Starter Schematic Diagram

Conventional Motor Starter Circuit Converted for PLC System Management