Programable Logic Controller questions. PLCS
1. (a) By using a truth table show that the expressions
are equivalent.
(b) Show by plotting truth tables that
NAND gate NOR gate inverter
2. (a) Draw the logic circuit to represent the following Boolean expression
using only NAND gates.
(b) Draw the logic circuit to represent the following Boolean expression
using only NOR gates.
Indicate the outputs of each gate.
F A B A B= + + +
F AB BC AC= . .
≡ ≡
ABC AC BC ABC+ + =
2
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3. In the ladder diagram shown in FIGURE 1, Y006 is a lamp, X and R
references are relay contacts. Write down the Boolean expression for the
lamp to light.
FIG. 1
4. (a) Add the following HEX numbers converting to binary. Express the
answer in HEX.
(i) 08 + 03
(ii) 0ABC + DA0E.
(b) Write down the 2's complement of:
(i) 00000001
(ii) 00111100
(c) Convert the following denary numbers to 8 bit binary numbers and
evaluate using the '2's' complement' method. Leave your answers in
binary form.
(i) 17 – 8
(ii) 97 – 125
(d) (i) Write down the bcd form of decimal 845.
(ii) Convert the denary number 818 first to hexadecimal then to
binary.
Show all working.
X001 X005 R001 Y006
X002 X003
R000
3
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5. State three disadvantages of relays when compared with the use of PLCs
in process control circuits.
6. (a) State what is meant by input and output interfaces in relation to
PLCs.
(b) State briefly what the function of interfaces is in relation to the
different types of process signal.
(c) The diagram of FIGURE 2 shows a bi-directional opto coupler input
interface circuit. When a supply voltage of 20 V is applied the LED
carries a current and 2 V is dropped across it. Calculate the value of
the LED current and the value of current through the 3 kΩ resistance.
FIG. 2
3kΩ
470Ω Signal
to micro-
computer
Bi-directional opto-coupler
Internal wiringExternal wiring
Separate supply used for inputs. Note: Either polarity.
Switch
4
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5
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(d) The circuit shown in FIGURE 3 is part of the interface of a relay
output module. Ib is 1 mA and VCC is 9 V. The relay requires a
minimum of 50 mA to energise.
Complete the values of the assumptions listed below in order to
calculate:
• voltage across R1
• value of R1
• voltage across the relay coil
• voltage across R2
• value of R2
• collector current Ic.
Assumptions:
Logic '1' = V
Logic '0' = V
Transistor forward current gain hfe =
LED current = 10mA
LED voltage drop at 10 mA = V
Base/emitter voltage = V
Collector emitter voltage when transistor is on = 1 V
FIG. 3
R
R 1
R 2
+V CC
N/O
External Terminals
Logic Signal
0 volts
6
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