A ferry terminal uses logic to decide whether to lower a vehicle barrier. Fig. 1 shows the logic circuit in its control cabinet. Input P is 1 when a vehicle...
Question 1 Report
A ferry terminal uses logic to decide whether to lower a vehicle barrier. Fig. 1 shows the logic circuit in its control cabinet. Input P is 1 when a vehicle has paid. Input E is 1 when a staff member uses an emergency override. Input M is 1 while the barrier is being repaired. Output B is 1 when the barrier can lower. The binary signals are stored in a test file before new code is installed.
Table 1 gives selected test data for the program.
| P | E | M | B |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 1 | 0 | 0 | 1 |
| 0 | 1 | 0 | 1 |
| 1 | 1 | 1 | ? |
(a) Identify the gate that combines P and E. [2]
(b) Complete the missing value of B in Table 1. [3]
(c) State why the NOT gate is used on input M. [2]
(d) Write a Boolean expression for B. [3]
Answer Details
(a) P and E enter an OR gate [2]. Thus either a paid vehicle or an emergency override can supply a 1 to the next gate.
(b) For P = 1 and E = 1, the OR output is 1. M = 1 passes through NOT, becoming 0. The final AND has inputs 1 and 0, so B = 0. [3]
(c) The NOT gate changes M = 1, meaning repairs are in progress, to 0 [1]. The AND gate then cannot output 1, so the barrier cannot lower during repair [1]. [2]
(d) The circuit is represented by:
\[B = (P \text{ OR } E) \text{ AND NOT } M\]
[3]
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