1. First, let’s pronounce the name correctly, what exactly is RS?
RS = Recommended Standard, which translates to “recommended standard” and is not a patent of any company. The number following it is the version number:
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232: Released in 1962, it’s the grandparent, mainly for computer COM ports;
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422: Released in 1975, it’s the uncle, focusing on long distances;
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485: Released in 1983, it’s the hot favorite, widely used in industrial sites.
2. A table to quickly understand the 3 core differences
| Dimension | RS-232 | RS-422 | RS-485 |
|---|---|---|---|
| Maximum Distance | 25 m | 1200 m | 1200 m |
| Maximum Speed | 20 kbps | 10 Mbps | 10 Mbps |
| Communication Method | 1 to 1 (point-to-point) | 1 to 10 (master-slave) | 1 to 32 (master-slave) |
| Working Mode | Full duplex | Full duplex | Half duplex (can be full duplex) |
| Signal Lines | 3 lines (TX/RX/GND) | 4 lines (TX+/TX-/RX+/RX-) | 2 lines (A/B) |
| Interference Resistance | Weak (single-ended) | Strong (differential) | Strong (differential) |
| Typical Levels | ±5~±15 V | ±2~±6 V | ±2~±6 V |
Remember the mnemonic: 232 short distance, one-to-one; 422 long distance, one-to-ten; 485 even longer, one-to-thirty-two.
3. Dissecting the interface, what is hidden in the 9 pins?
Many people get confused when they see DB9, so let’s use a diagram:
| PIN | RS-232 | RS-422 | RS-485 |
|---|---|---|---|
| 2 | RXD (Receive) | TX- (Negative Send) | Not fixed |
| 3 | TXD (Send) | TX+ (Positive Send) | Not fixed |
| 5 | GND | GND | GND |
| 4/7/8 | Flow Control RTS/CTS | RX+/RX- | Unused |
Why does 485 only have 2 wires A/B, yet it can still work? The answer: differential! A-B ≥ +200 mV is logic 1, A-B ≤ -200 mV is logic 0. The two wires twisted together cancel out noise, greatly enhancing interference resistance.
5 Major Pitfalls on Site, Each One Costs You Dearly
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Plugging 232 into a 485 port causes smoke 232 levels ±15 V, 485 chip withstand voltage ±7 V, plugging it in will cause immediate failure.→ Always confirm the interface definition before powering on.
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Forgetting to add terminal resistors to the 485 bus 1200 m long line signal reflection at the end, causing data corruption.→ Place a 120 Ω resistor at both ends to solve the problem.
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Reversing A/B connections All devices go offline, only one can communicate.→ Standardize colors: A=green, B=white, no more conflicts on site.
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Connecting 232 three wires directly, with ground floating Two computers seem to connect, but actually lose 50% of packets.→ Ground must be common, don’t let the signal drift away.
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Using 422 full duplex as 485 half duplex Result: master sends, all slaves go silent.→ Check the chip model: MAX485 is only half duplex, MAX491 is full duplex.
Selection Mnemonic: When the boss asks how to choose, one sentence to summarize
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Within 3 meters of the computer: 232, cheap and easy;
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Within 100 meters, one-to-one: 422, stable against interference;
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Over 100 meters, one-to-many: 485, industrial standard;
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Need full duplex and long distance: use 422 with optical isolation module, price triples, but stability is guaranteed.
A real case, a 485 bus saved the entire factory production line
Client site: 12 frequency converters, 800 m apart, Modbus RTU.Problem: Random disconnections every day, rebooting helps.Investigation steps:
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Using an oscilloscope to check the waveform, found that the signal at the 800 m end was only 0.8 V;
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Added a 120 Ω terminal resistor, signal increased to 1.5 V;
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Replaced the main line with 0.75 mm² twisted shielded wire, signal directly at 2.8 V;
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Added a 485 isolation module at the master station, common mode interference dropped from 2 V to 0.2 V.Result: Continuous operation for 3 months with 0 failures, the boss treated me to barbecue.
Don’t let 232/422/485 confuse you anymore. Remember the mnemonic: 232 point-to-point, 25 m; 422 full duplex, 1200 m; 485 half duplex, one-to-thirty-two. Next time someone asks on site, just throw this article at them!
If you find this useful, please like and share to help more industrial control people avoid detours~ More knowledge will be shared later, remember to follow!
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