Chapter 12, Problem 1RQ

Program Plan Intro

Sequencers:

• Programmable Logic Controller (PLC) sequencer performs ON or OFF output patterns using sequencer instructions.
• The sequencer output instruction is used for controlling the output devices.
• PLC sequencer instructions are used for controlling machines which provides a stepped sequence of repeatable operations.
• It simplifies the ladder program by allowing the user to use a single or pair of instructions for performing complex operations.
• Various sequencer instructions can be programmed based on the PLC manufacturer.

Drum switch:

• Drum switch is also known as sequencer switch.
• The drum switch includes a series of normally open contact blocks which are operated by pegs situated on a motor-driven drum.

Explanation of Solution

Operation of a drum switch:

The operation of a drum switch is described as given below.

• In a drum sequencer, the pegs are situated at specific locations around the circumference of the drum for operating the contact blocks.
• When the drum starts rotating, no pegs will remain open and all the contacts that are arranged in a line with the pegs will remain closed.
• Logic 1 or ON is used to indicate the presence of a peg and logic 0 or OFF is used to indicate the absence of a peg.

The mechanical drum-operated sequencer switch is depicted as follows.

• The ON/OFF operation of 16 discrete outputs is controlled by the drum cylinder.
• The data table given below is used to illustrate the logic state for the first four steps of the drum cylinder.

1	1	1	1	0	0	0	1	0	0	0	1	0	1	0	1
0	0	0	0	1	1	1	0	1	0	1	0	0	1	0	1
0	0	0	0	0	0	1	0	1	0	0	0	1	1	1	0
0	1	1	1	0	0	0	0	0	1	0	1	0	1	0	1

Here, each location where a peg is present is represented by 1 (ON) and the locations where there are no pegs are represented by 0 (OFF).