RLL Relay Ladder Logic CONTENTS 1 PLC operation

RLL: Relay Ladder Logic CONTENTS 1. PLC operation 2. PLC programming 3. Ladder logic 4. Memory and gates 1

PLC operation A PLC has 2 modes of operation Programming mode: translate engineering language (control logic) to machine language (binary code) Running (or scanning) mode: Relating the program to inputs and outputs 2

Programming through standard computer n Most PLC manufacturers offer software packages that allow a standard computer to be used as a programming terminal 3

PLC running or scanning mode Relating the program to inputs and outputs n The CPU reads the data from the inputs n The program in the CPU uses the inputs to evaluate the control logic. As the program runs, the CPU updates the data n The CPU writes the data to the output 4

Relating the program to inputs and outputs 5

PLC programming n One of the advantages of PLC is that it can be programmed by non-specialists n Program can be written in the form of a relay ladder diagram represented in terms of contacts and coils. n Contact: A simple input switch. n Coil: An output load, e. g. , a relay or motor. current Switch/contact coil 6

Ladder logic current n n n Switch/contact coil Power supply rails drawn as parallel vertical lines on left and right Connection of rails implies current will flow An output is “on” when a connection is completed and current flows through the load’s coil

Illustration of ladder logic Power circuit Control circuit Switch Relay Coil Simple control circuit of a bell Associated Relay Ladder 8

Ladder logic: basic notations n Simple “always on” load: [Always_On = 1] note: sometimes illegal n Load controlled by a single contact: [Switch_Con = Switch]

Ladder logic: AND gate, OR gate n Boolean logic C = A and B also noted C = A. B C = A or B also noted C = A+B

Ladder logic: NOT gate n n C = (not A) also noted Ā A contact with a slash through it is “normally closed. ” This indicates a connection when A is NOT triggered. So when sensor/input A is activated, there is an open circuit

Ladder logic: basic operations n n Each rung of the ladder is a statement that is asynchronous when implemented in relay logic, but evaluated sequentially by the PLC. X = (A+B). (C+D), Y = Ā. [B+(C. D)]

Ladder logic: examples Lamp = SW 1 + SW 2 13

Ladder logic: setting a memory S 1 Q 1. 1 1 Relay Coil “R 1” Motor Current flow (Scan 1) S 1 Q 1. 1 Motor starter 2 Current flow (Scan 1) Output Q 1. 1 Current flow (Scan 2) S 1 Q 1. 1 PLC 3 Input Q 1. 1 Current flow (Scan 2) Switch “S 1” to start 14

Setting memory elements using AND and OR gates S 1 S 2 S 3 R 1 Memory Unit AND Gate for setting S 3 R 1 Memory Unit OR Gate for setting R 1 S 2 R 1 15

Ladder logic: resetting a memory S 1 1 S 2 Q 1. 1 Relay Coil “R 1” Motor Q 1. 1 S 2 Q 1. 1 Motor starter 2 Q 1. 1 S 1 Output S 2 Q 1. 1 PLC 3 Q 1. 1 Input Switches “S 1” to start “S 2” to stop 16

Resetting memory elements using AND and OR gates S 2 S 1 R 1 S 3 R 1 S 2 S 3 S 2+S 3 NOT(S 2+S 3) 0 0 0 1 1 0 1 0 1 1 1 0 The only case for current ON is when (S 2 is NOT switched ON) AND (S 3 is NOT switched ON) 17

Resetting memory elements using AND and OR gates R 1 S 2 S 1 S 3 R 1 S 2 S 3 S 2. S 3 NOT(S 2. S 3) 0 0 0 1 0 1 1 0 The only case for current OFF is when (S 2 is switched ON) AND (S 3 is switched ON) 18

Ladder logic: adding indicators as outpu S 1 S 2 R 1 R 1 Q 1. 1 G R 19

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