Page 78 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Unit 4: Minimization of Boolean Algebra

A circuit designer is free to make the output for any “don’t care” condition either a 0 or Notes
a 1 in order to produce the simplest output expression. For example the K map for this
truth table is shown in Figure 4.24 (b) with an x placed in the ABC and ABC squares.
The designer here would be wise to change the x in the ABC square to a 1 and the x in the
ABC square to a 0, since this would produce quad that can be looped to produce z = A,
as shown in Figure 4.24 (c).
Whenever “don’t care” conditions occur, we have to decide which one change to 0 and
which to 1 to produce the best K-map looping (i.e. the simplest expression). This decision
is not always an easy one.

Press Wiring

n electrical layout is needed for a hydraulic press. The press uses a 24Vdc double
actuated solenoid valve to advance and retract the press. This device has a single
Acommon and two input wires. Putting 24Vdc on one wire will cause the press to
advance, putting 24Vdc on the second wire will cause it to retract. The press is driven by a
large hydraulic pump that requires 220Vac rated at 20A, this should be running as long as the
press is on. The press is outfitted with three push buttons, one is a NO stop button, the other
is a NO manual retract button, and the third is a NO start automatic cycle button. There are
limit switches at the top and bottom of the press travels that must also be connected.

The input and output cards were both selected to be 24Vdc so that they may share a single
24Vdc power supply. In this case the solenoid valve was wired directly to the output card,
while the hydraulic pump was connected indirectly using a relay (only the coil is shown for
simplicity). This decision was primarily made because the hydraulic pump requires more
current than any PLC can handle, but a relay would be relatively easy to purchase and install
for that load. All of the input switches are connected to the same supply and to the inputs.

Questions:
1. Explain the need of hydraulic press.
2. What are limit switches at the top and bottom of the press?

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