Free RS-485 Bus Termination & Bias Calculator
Size line termination resistors (120Ω) and fail-safe pull-up/pull-down bias networks. Prevent floating UART frame errors and maintain +200mV differential noise margin.
🖧 Bus Topology & Operating Conditions
Bus Line Termination: 120 Ω at both extreme ends
• Line A (Data + / D+): Pull UP to V_CC via 560 Ω.
• Line B (Data - / D-): Pull DOWN to GND via 560 Ω.
• Install at 1 Node Only: Always place fail-safe bias network at only ONE central node or master end to avoid overloading drivers.
The Floating UART Glitch: When all RS-485 transceivers release the bus into high-impedance mode, line noise causes $V_A - V_B$ to drift near 0V. Standard receivers oscillate or detect a false start-bit, triggering garbage byte interrupts!
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Tested hardware and components for high reliability
The Three Fail-Safe Operating States
The TIA/EIA-485 standard defines a receiver's output:
- Logic High (1): Differential voltage $V_A - V_B > +200 ext{ mV}$.
- Logic Low (0): Differential voltage $V_A - V_B < -200 ext{ mV}$.
- Indeterminate State: Between $-200 ext{ mV}$ and $+200 ext{ mV}$, receiver behavior is undefined.
Mathematical Sizing Formulas
Receiver Input Load: R_rx = (12,000 / UL) / N_nodes
Total Differential Load: R_diff = (R_term × R_rx) / (R_term + R_rx)
Bias Resistor (Each): R_bias = ((V_cc - V_AB) / (2 × V_AB)) × R_diff
Why Stub Length Must Be Minimized
Never wire RS-485 in a star, tree, or ring topology! Always use a daisy-chain bus. Stubs branching off the main trunk line must be strictly shorter than one-quarter of the signal rise-time electrical length (typically $< 30 ext{ cm}$ / 1 foot at 10 Mbps).
Frequently Asked Questions
Why must termination resistors only be installed at the two ends of the bus?
Installing termination resistors at every node in a multi-drop network places them all in parallel. If 10 nodes each had 120 Ohm resistors, the parallel bus load would collapse to 12 Ohms, drawing over 400 mA and burning out the transmitter output stage. Only the two physical ends of the cable trunk should be terminated.
What is a True Fail-Safe transceiver (e.g. MAX3485 or SN65HVD72)?
Modern transceivers have redesigned internal differential comparator thresholds (e.g. -200 mV to -50 mV instead of +/-200 mV). When the bus is open, shorted, or floating at 0V, the receiver output is guaranteed to clamp to logic High (1) without needing external bias pullup resistors.
Can 120 Ohm RS-485 cable be mixed with 100 Ohm Cat5 cable?
Mixing cable impedances causes continuous reflections at every splice junction. If using Cat5/Cat6 network cable, terminate with 100 Ohm resistors at each end to match the cable characteristic impedance.