RS-232C Logic 1 Voltage Explained | Generated by AI
Question: What is the voltage level for logic “1” in the RS-232C electrical specification?
Answer:
RS-232C Logic “1” Voltage Level — Correct Answer: D (-15V ~ -5V)
Key Electrical Characteristics of RS-232C
RS-232C uses an inverted (negative logic) voltage scheme for data transmission lines, which is counterintuitive compared to most digital logic standards.
| State | Driver Output Voltage | Receiver Threshold |
|---|---|---|
| Logic “1” (MARK) | -5V to -15V | -3V to -15V |
| Logic “0” (SPACE) | +5V to +15V | +3V to +15V |
| Undefined / Invalid | -3V to +3V | — |
Why the Answer is D, Not C
The question specifically asks about the driver output voltage requirement:
- Drivers must supply -5V to -15V for a logic 1, and +5V to +15V for a logic 0.
- Taking the 2V noise margin into account, a low level (-3V to -15V) is defined as a logic 1 (marking) at the receiver input.
So the distinction is:
- Driver output (what the transmitter sends): -5V to -15V → This is Answer D
- Receiver threshold (what is accepted as valid logic 1): -3V to -15V → This corresponds to Option C
Option C (-13V to -3V) describes the receiver detection range (with noise margin), not the driver output requirement. The exam answer D (-15V ~ -5V) refers to the driver’s required output voltage to properly signal logic “1.”
Summary
RS-232C uses negative voltage for logic “1” (called “MARK”) and positive voltage for logic “0” (called “SPACE”). The driver must output between -5V and -15V to represent a valid logic “1”. This is the opposite of standard TTL/CMOS logic.
References:
- Fundamentals of RS-232 Serial Communications – Analog Devices
- RS-232 – Wikipedia
- Understanding the RS-232 Standard – DigiKey