Chapter 6 Introduction to Logic

Objectives Explain AND logic. Describe OR logic. Explain NOT logic. Explore a truth table for specified logic. Determine if a PLC ladder rung is true or false under specified conditions.

Hardwired Electrical Circuit Has Electrical Continuity Hardwired circuit has actual current flow. This is called electrical continuity. Current flows through switching device to directly control the load.

Hardwired Relay Circuit

Logical Continuity Hardwire relay circuits have electrical continuity, or current flow. PLCs do not operate on electrical continuity, but logical continuity.

Logical Continuity (cont’d.) A PLC ladder program resembles an electric schematic. A PLC program is a set of instructions stored in memory. PLC ladder symbols represent ladder program instructions.

Logical Continuity (cont’d.) PLC instructions examine the status of a bit at the associated address. Bits are either a 1 or 0. Instructions are either true or false. PLCs have logical continuity.

Signal Flow into and out of a PLC

Input Signal Seen by Input Module Signal from field device is wired to input module screw terminals. These signals are called input signals or simply inputs. Each input point has a unique identifier called an address.

Input Module Isolates and Converts Discrete input module determines if input is a valid ON signal. The discrete input module converts the incoming electrical signal to +5v DC, or 0v DC. An ON state will be converted to +5v DC. An OFF state will be converted to 0v DC.

Input Section to Back Plane The high voltage input signal is isolated from the lower voltage microprocessor and supporting circuitry in the processor. Signal is then transferred to the chassis back plane and sent to processor memory for storage.

Processor and Input Status File Field input signals are transferred from the input module by way of the back plane for storage in the processor’s input status file. There is one storage location in the status table for the on or off status of each input device. On signal stored as a 1, off as a 0.

Processor Examines Status Table Bit When running, the processor solves ladder instructions by examining the on or off status of the input device by examining the associated memory location for a 1 or 0.

Ladder Program Solved Ladder instructions have rules regarding when they are true or false. Instructions are made true or false as a result of the examined bit. If there is a path of true input instructions to the output instruction, the rung will be true.

Output Update When all ladder rungs have been solved, the output status table data is sent to the respective output modules one word at a time. When rung is solved, the output status table is updated.

Combining and Solving Ladder Instructions Three basic types of logic: And Or And / Or (combinational)

Series Logic When two switches are wired in series, both switches must be ON before power will flow. Switch 1 AND switch 2 must all pass power before light 1 will turn on.

Conventional Series Circuit

Switch 1 AND Switch 2 Need to Be Closed to Energize L-1

Truth Table A truth table shows all possible input conditions and the expected output conditions. The following slide shows a truth table for AND logic.

Truth Table for AND Logic

PLC Instructions in Series PLC instructions work the same basic way as hard wired field devices. Instructions in series are considered ANDed together, or AND logic. Switch 1 AND switch 2 must be on for L-1 to come on.

PLC Representation of a Rung

Truth Table for Two-Input AND Logic

Three-Input Series Circuit

Three-Input PLC Ladder Rung

Three-Input AND Logic Truth Table

Parallel Is also OR Logic Inputs in parallel are said to be ORed. Switch 1 is ORed with switch 2. If switch 1 is closed OR switch 2 is closed, light 1 will turn on. If both SW1 and SW2 are closed, light 1 will turn on.

Conventional Parallel Circuit

Programmable Controller Ladder Diagram

Two-input OR Truth Table

Truth Table for NOT Logic

Conventional Ladder Diagram

PLC Representation of N.O and N.C. Instructions All PLCs use the standard normally open and normally closed instructions. Different manufacturers use different names for the instructions.

Normally Open and Normally Closed Instruction Identification

Combination AND-OR Logic Input devices can be combined in series (ANDed) and also in parallel (ORed) on a PLC ladder rung. Instructions programmed to provide the proper input conditions for your application. This is called combination AND-OR logic.

Combination AND-OR Logic