In asynchronous serial communication, what is the role of "Hardware Handshaking" (e.g., using RTS/CTS lines)?
- (a)To synchronize the internal clocks of both devices
- (b)To encrypt data before transmission
- (c)To provide error detection using parity bits
- (d)To prevent data loss by managing buffer overflow
Answer
Why
Correct — D. Hardware handshaking is flow control on dedicated wires. The receiver uses the RTS/CTS lines to signal whether it can accept more data. When its buffer is close to full, it drops the signal and the sender pauses until it returns.
No byte arrives with nowhere to go, which is option (d): preventing data loss from buffer overflow.
Why the others are wrong
- (a)To synchronize the internal clocks of both devices — Asynchronous links have no shared clock at all. Each character begins with a start bit and both ends are set to the same baud rate in advance, so RTS/CTS has no clocks to synchronise.
- (b)To encrypt data before transmission — RTS and CTS are ready / not-ready control signals. They never touch the data bits, so they cannot encrypt anything.
- (c)To provide error detection using parity bits — Parity is a separate bit inside each character frame and a simple form of error checking. It is configured on its own and works whether handshaking is on or off.
Concept
Flow control stops a fast sender from overrunning a slow receiver. In hardware flow control the stop and go messages travel on extra wires, RTS (Request To Send) and CTS (Clear To Send), not inside the data stream.
Windows' serial driver documents one concrete version. With RTS handshaking on, it lowers RTS when its input buffer is more than three-quarters full and raises it again below half full. With CTS monitoring on, it suspends output while CTS is off.
The alternative is software flow control, XON/XOFF, where the receiver sends special stop and start characters inside the data stream. Windows' serial settings offer both.
Key facts
- RTS stands for Request To Send and CTS for Clear To Send.
- Asynchronous serial transfers data without an external clock signal, and a start bit marks the beginning of each character.
- With CTS flow control enabled, Windows' serial driver suspends output while CTS is off.
- XON/XOFF is software flow control: stop and start characters sent within the data.
Study next
Common traps
- Reading 'asynchronous' and assuming the handshake must synchronise clocks, when asynchronous links share no clock.
- Confusing flow control, which decides when to send, with parity, which checks whether a character arrived intact.
The stem names the mechanism, RTS/CTS, and asks what it is for. Two wrong options are jobs done by other parts of a serial link, timing (the start bit and agreed baud rate) and error checking (the parity bit).
Related PYQs
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