signal-differential-dat
board
🔍 Key Differences: RS-485 vs RS-422
| Feature | RS-485 | RS-422 |
|---|---|---|
| Driver count | Multiple drivers | One driver only |
| Receiver count | Up to 32 receivers | Typically up to 10 |
| Duplex mode | Half or full duplex | Full duplex (typical) |
| Biasing required | ✅ Yes (often) | ❌ Not usually needed |
| Termination | ✅ Yes (120Ω) | ✅ Yes (120Ω) |
🧠 MAX490 in RS-422 Setup
- The MAX490 can act as the driver or receiver in an RS-422 setup.
- Ensure only one driver is active on the bus.
- Communication is half-duplex only, due to MAX490 design.
- For full-duplex RS-422, use a different transceiver (e.g., MAX491).
MAX490 for RS422
| Feature | RS-422 (with MAX490) | Notes |
|---|---|---|
| Driver | 1 (only one MAX490 TX active) | Matches RS-422 spec |
| Receiver | Up to 32 (MAX490 supports 32 RX) | RS-422 typically supports 10 |
| Termination needed | ✅ Yes | 120 Ω between A and B |
| Biasing resistors needed | ❌ Usually not | Optional; only if line floats |
| Duplex Mode | Half-duplex only | MAX490 can't do full-duplex RS-422 |
✅ Termination Resistor:
- Use a 120 Ω resistor between A and B at the end of the cable.
❌ Biasing Resistors:
- Not required in RS-422 because the single driver maintains a known state.
- Optional if you observe floating voltages or noise.
Pull-up and Pull-down Resistors for MAX490 (RS-485)
The MAX490 is a differential RS-485/RS-422 transceiver. While it does not require internal pull-up or pull-down resistors, external resistors are typically used for bus biasing to ensure known voltage levels when no device is actively driving the bus.
✅ When Are They Needed?
- When no device is guaranteed to drive the RS-485 bus (e.g., during idle periods).
- To avoid undefined logic levels on the differential lines A and B.
- Especially important in multi-drop configurations.
🛠️ Recommended Biasing Setup
Typical resistor values:
- Pull-up on A: 680 Ω to +5V
- Pull-down on B: 680 Ω to GND
- Termination resistor: 120 Ω between A and B (at both ends of the bus)
📘 Circuit Diagram
+5V
|
R1 = 680Ω
|
A -----------+------------------------------+--- RS-485 Bus
| |
Rterm = 120Ω Other devices
| |
B -----------+------------------------------+--- RS-485 Bus
|
R2 = 680Ω
|
GND
🔍 Notes
- MAX490 includes receiver failsafe features, but external biasing is still standard practice.
- In point-to-point setups, biasing might be optional but is still recommended for stability.
- In multi-drop (multi-device) RS-485 networks, biasing is essential for reliable communication.
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