Serial communications basics: what is the minimum number of conductors required to send data in one direction over a simple serial link (assume asynchronous RS-232-style signaling and no hardware flow control)?

Introduction:
Real-world serial links are built from conductors that carry data and reference signals. The absolute minimum you need depends on directionality (simplex vs. full duplex) and whether you rely on hardware handshaking lines. This question asks for the minimum number of wires to send data in one direction over a simple asynchronous serial connection like RS-232, with no hardware flow control.

Given Data / Assumptions:

• Simplex one-way transmission (source to sink only).
• Asynchronous character framing (start/stop bits) removes the need for a separate clock wire.
• No RTS/CTS, DTR/DSR, or other hardware flow-control leads are used.
• Ground reference is required for reliable signaling.

Concept / Approach:
In asynchronous RS-232-style links, the transmitter places a voltage on the TxD conductor relative to a common signal ground. Because the receiver needs a reference, at least one return/ground conductor must be present. Therefore, for one-way data transmission, two conductors suffice: TxD and signal ground. Full-duplex requires a third for RxD, and hardware flow control adds more wires, but these are not strictly required to send data one-way.

Step-by-Step Solution:

Verification / Alternative check:
Practical lab tests with USB-to-TTL or RS-232 adapters confirm that a two-wire hookup (Tx and GND) can send characters; adding Rx enables full duplex; adding RTS/CTS introduces hardware flow control but is optional for the minimal link.

Why Other Options Are Wrong:

• 1 wire: lacks a reference ground, leading to undefined voltage levels.
• 4 or 6 wires: include lines that are helpful (Rx, RTS/CTS, DTR/DSR) but not strictly necessary for one-way sending.
• None of the above: incorrect because two wires are sufficient.

Common Pitfalls:
Assuming full duplex or hardware flow control is mandatory; forgetting that asynchronous serial embeds timing with start/stop bits, avoiding a dedicated clock wire.

Final Answer:
2 conductors (TxD and signal ground).