PLC Programming. Ladder Diagrams

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Abel Boyd
5 years ago
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1 Ladder Diagrams Consider the diagram below showing a circuit for switching an electric motor on or off. We can redraw this diagram in a different way, using two vertical lines to represent the input power rails and connecting the rest of the circuit between them. L Power Supply N Switch M Motor L Switch Motor N Electrical Circuit Ladder Diagram The power lines or rails are vertical lines with the horizontal lines like rungs of a ladder.

2 Ladder Programming In drawing a ladder diagram the following conventions are adopted:. Each rung on the ladder defines one operation in the process control. 2. The ladder diagram as read from left to right from top to bottom. 3. Each rung must start with an input/s and must end with an output/s. 4. Electrical devices are shown in their normal condition. 5. particular device can appear in more than one rung of a ladder. 6. The inputs and outputs are all defined by there addresses. 2

3 Ladder Logic Symbols Normally open input contacts Normally closed input contacts Output device END The end rung 3

4 Logic Functions There are many control situations requiring actions to be initiated when certain combination of conditions is realized. For example a drill machine is only to be started when work piece is present (interlock). ND function INPUTS OUTPUTS Supply Voltage Load Input ND Output Input Input Output I:/ I:/ O:/ 4

5 OR function INPUTS OUTPUTS Supply Voltage Load Input OR Output Input Output I:/ Input O:/ I:/ 5

6 NOT function Input Output Load NOT INPUTS OUTPUTS Supply Voltage Input Output I:/ O:/ 6

7 NND function INPUTS OUTPUTS Input Output Supply Voltage Load NND Input Input Output I:/ I:/ O:/ 7

8 NOR function Supply Voltage Load Input NOR Output INPUTS OUTPUTS Input Output I:/ Input O:/ I:/ 8

9 XOR function Input Input Output O:/ INPUTS OUTPUTS Input Input NOT ND ND OR NOT 9

10 Latching There are often situations where it is necessary to hold an output energized, even when the input ceases. Input Input Output Output O:/ O:/

11 Latching example I:/ I:/ O:/ Motor O:/ O:/ O:/ Stop Light O:/ O:/2 Run Light END

12 Instruction Lists nother PLC programming method, which can be considered to be the entering of a ladder program using test, is instruction lists. For this, mnemonic codes are used, each coed corresponding to a ladder element. Code LD LDN ND NDN OR ORN OR N ST Description Start a rung with open contact Start a rung with a closed contact series element with open contact series element with closed contact parallel element with open contact parallel element with closed contact ranch parallel elements Connect branch elements in series n output 2

13 Instruction Lists Example: Input Input Output I:/ I:/ O:/ Step Instruction LD I:/ 2 ND I:/ 3 ST O:/ 3

14 Instruction Lists Exercise: Write the instruction list for the following ladder diagrams: (a) I:/ O:/ I:/ (b) I:/ O:/ I:/ 4

15 Instruction Lists Exercise: Write the instruction list for the following ladder diagrams: I:/ I:/ O:/ (c) O:/ (d) I:/ I:/ O:/ I:/ I:/ 5

16 Function lock Diagrams asically types of diagrams used to describe logic systems. function block is a program instruction unit which, when executed, yields one or more output values. Q >= Q Ladder Diagram Equivalent function block diagram 6

17 Function lock Diagrams Q & Q Ladder Diagram Equivalent function block diagram 7

18 Function lock Diagrams Q Q >= & Q Ladder Diagram Equivalent function block diagram 8

19 Function lock Diagrams Exercise: Draw the function block diagram for the following ladder programs: a) C D Q E b) C D G Q Q E F 9

20 Internal Relays In PLC s there are elements that are used to hold data, i.e. bits, and behave like relays, being able to be switched on or off and switch other devices on or off. Such internal relays do not exist as real world switching devices but are merely bits in the storage memory that behave in the same way as external relay output and input. There are several terms used to describe these elements, i.e. uxiliary relays, Flags, Markers, Coils. 2

21 Internal Relays In In 2 IR In 3 IR In 4 Out 2

Lec 2. .Ladder Logic.Memory Map.KGL Programming Software

Lec 2. .Ladder Logic.Memory Map.KGL Programming Software Lec 2.Ladder Logic.Memory Map.KGL Programming Software Ladder logic is the main programming method used for PLCs. Relay is a simple device that uses a magnetic field to control a switch, when a voltage