---
title: "RS-422A vs RS-232C distance and speed"
subject: "Instrumentation II"
chapter: 3
key: "semester_5/instrumentation_ii:rs422-vs-rs232-distance"
source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/rs422-vs-rs232-distance
site: BCT Engineering
---

# RS-422A vs RS-232C distance and speed

Instrumentation II · Chapter 3: Serial Interfacing With Microprocessor Based System

- Asked in: 2073 Shrawan, 4 marks; 2073 Chaitra, 1 marks; 2070 Asar, 2 marks (asked 3 times)
- Video: [Difference between RS 485 RS 422 | RS 232 | LVDS | M-LVDS](https://www.youtube.com/watch?v=Ndgh7Pv4oYc) — Tech scr
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/rs422-vs-rs232-distance
- Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-3

## Question

Explain why a system that uses the RS 422A can transmit data over longer distance and at higher baud rate than RS 232C and RS 423A.

## Short answer

RS-422A uses balanced differential signaling on twisted pairs, canceling common-mode noise and ground shifts, while RS-232C is single‑ended and ground‑referenced. Differential receivers detect only 200 mV difference, and terminated lines prevent reflections, enabling 10 Mbps over 1200 m, vastly better than RS-232C’s 20 kbps and 15 m.

## Answer

RS-422A uses **balanced differential signalling** on twisted pairs, whereas RS-232C and RS-423A use **single-ended (unbalanced) signalling** referred to a common ground. This is the reason for its longer distance and higher speed.

#### Reasons
1. **Noise immunity:** each signal travels on two wires with opposite polarity (A and B). The receiver is a differential amplifier, so noise picked up equally on both wires (common-mode noise) cancels. In RS-232C the noise adds directly to the single signal wire.
2. **Ground potential difference:** in RS-232C/423A the signal is measured against a shared ground, so a ground voltage difference between distant devices causes errors. In RS-422A the receiver senses only the difference between the two wires, so ground shifts (up to several volts) are tolerated.
3. **Low crosstalk and radiation:** the currents in the pair are equal and opposite, and the twisting reduces coupling.
4. **Terminated transmission line:** a 100 ohm terminator avoids reflections, so fast edges can be used on long cables. RS-232C lines are unterminated and the cable capacitance is limited to 2500 pF, which limits the length to about 15 m.
5. **Sensitive receiver:** it detects a difference as small as 200 mV, so the signal can weaken over a long cable and still be received.
6. **Strong line drivers** with low output impedance and high drive current.

| Parameter | RS-232C | RS-423A | RS-422A |
|---|---|---|---|
| Signalling | Single-ended | Single-ended, differential receiver | Differential |
| Max. data rate | 20 kbps | 100 kbps | 10 Mbps |
| Max. distance | about 15 m | 1200 m at 1 kbps (90 m at 100 kbps) | 1200 m at 100 kbps (12 m at 10 Mbps) |
| Receivers | 1 | 10 | 10 |
| Voltage levels | +/-3 to +/-15 V | +/-4 to +/-6 V | +/-2 to +/-6 V differential |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What kind of signalling does RS-422A use that RS-232C does not?
   A. Balanced differential on twisted pairs
   B. Single-ended with common ground
   C. Current loop only
   D. Optical signalling
   Answer: A. RS-422A is balanced differential; RS-232C and RS-423A are single-ended. (Key point: Signalling type)

2. Why does RS-422A reject noise?
   A. It uses higher voltage
   B. Noise appears equally on both wires and cancels in the differential receiver
   C. It uses shorter cables
   D. It has no ground
   Answer: B. Common-mode noise is identical on both wires, so the difference amplifier removes it. (Key point: Common-mode noise)

3. How does RS-422A tolerate ground potential difference between distant devices?
   A. It needs a ground wire
   B. Receiver senses only the difference between two wires
   C. It uses 15 V levels
   D. It slows the baud rate
   Answer: B. Single-ended receivers measure against shared ground; RS-422A senses wire difference, tolerating ground shifts. (Key point: Ground difference)

4. Which is RS-422A's maximum data rate per the table?
   A. 20 kbps
   B. 100 kbps
   C. 1 Mbps
   D. 10 Mbps
   Answer: D. RS-422A reaches 10 Mbps (12 m) and 1200 m at 100 kbps. (Key point: Speed/distance)

## Related in Instrumentation II

- [RS-232C importance; time to transmit a string at 4800 baud](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ars232c-string-time-4800) — Instrumentation II, chapter 3
- [Compare RS-232C, RS-422A and RS-423A](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ars232-422-423-compare) — Instrumentation II, chapter 3
- [Bit rate, baud rate and character time at 9600](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Abaud-time-engineer) — Instrumentation II, chapter 3
- [RS-232C handshaking signals](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ars232-handshake-signals) — Instrumentation II, chapter 3, asked 2×

## Related in other subjects

- [T/F: more circuits rather than higher voltage](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-circuits-rather-than-voltage) — Transmission and Distribution System Design, chapter 2