Assertion (A) : In serial communication system, when the transmission of data goes in both ways, it is called full-duplex system. Now if two micro processors are connected in full duplex mode, the amount of data transmitted will be double to the amount of data in half-duplex mode connection. Reason (R) : When the transmission of data goes in one way, it is called half-duplex system and when the data moves in both ways, it is called full duplex system.
(A) is false, but (R) is true.
(A) is false, but (R) is true — option (D).
The reason is true and is simply the standard definition :
| Mode | Direction of transfer | Example |
|---|---|---|
| Simplex | One way only, never reversed | Broadcast radio, keyboard to computer |
| Half duplex | Both ways, but only one at a time | Walkie-talkie, older RS-485 buses |
| Full duplex | Both ways simultaneously | Telephone, RS-232 with separate TxD and RxD lines |
The assertion opens correctly and then overreaches. Its first clause — transmission going both ways is called full duplex — is right. The claim that fails is the second: that connecting two microprocessors in full duplex makes the amount of data transmitted double what half duplex would carry. That is an idealisation, not a fact, and it breaks down for several independent reasons :
| Why the doubling does not follow | Explanation |
|---|---|
| It assumes traffic is symmetric | Doubling requires both directions to be busy at once. Most real exchanges are request-and-response — one side waits while the other talks — and an idle return channel carries nothing |
| Half duplex is not half-rate | A half-duplex link uses the whole channel in whichever direction it is transmitting. Over a period it interleaves the two directions rather than halving each |
| Protocol overhead differs | Full duplex removes turnaround delay and acknowledgement waits, so the realised gain is usually better than a naive halving in interactive traffic and worse than double in bulk one-way transfer |
| The bottleneck may be elsewhere | Processor speed, buffer size or baud rate can limit throughput before the duplex mode does |
So (A) states a definition correctly and then attaches an unsupported quantitative claim to it. A statement that is right in part and wrong in part is false as a whole, which is what makes the code (D).
The lesson for assertion-reason items. Where an assertion contains two clauses, both must hold. Examiners construct these by pairing a familiar true definition with a plausible-sounding numerical consequence, relying on the candidate to accept the second because the first is obviously right. Read to the end of the sentence before judging it.
Where this appears in hardware: the 8085 family uses the 8251A USART, whose separate transmitter and receiver sections allow genuine full-duplex working; the mode is chosen by the mode instruction word written during initialisation.
Hence, the answer is (A) is false, but (R) is true.
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