RS-485 Bias & Termination Calculator

Calculate RS-485 bus bias resistors and termination for idle-line noise immunity. Ensures receiver sees valid differential signal during bus idle.

Calculator Electronics Updated Apr 23, 2026
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How to Use
  1. Enter bus VCC (typically 3.3 or 5 V), required idle differential voltage, and number of nodes.
  2. Tool computes bias resistor pair to maintain idle voltage above receiver threshold.
Input
V
mV
Ω (each end)
Presets
Bias Network
R_bias
—
Idle V_diff
—mV
Parallel Z
—Ω
Bias Current
—mA

Show Work

Enter values.

Formulas

Bias goal
V_A − V_B ≥ 200 mV idle
Typical failsafe threshold.
Bias R pair
R ≈ (VCC·Zterm − V_diff·Zterm/2) / (2·V_diff)
Pull-up on A, pull-down on B.
Typical
500-750 Ω each
5 V bus, 120 Ω term.
Parallel Z
Z_term || 2R = loaded Z
Drops below 120 Ω; keep ≥ 54 Ω per TIA-485.
Bias I
VCC / (2R + Zterm)
Continuous idle current.
Failsafe IC
No external bias needed
Modern transceivers.

History of RS-485

EIA RS-485 was standardized in 1983 as an extension of RS-422 for multi-drop industrial networks - up to 32 transceivers on a single twisted pair, 4000 ft cable, 10 Mbps. Modbus RTU (Modicon, 1979), Profibus (1989), and Theater's DMX512 (1986) all standardized on RS-485 physical layer. The need for idle-state biasing was addressed in TIA-485-A (2003) with the failsafe receiver requirement. Modern RS-485 transceivers include internal failsafe biasing, eliminating external bias resistors for most applications.

About This Calculator

Enter VCC, required idle differential voltage (typically ≥ 200 mV for RS-485 failsafe threshold), and termination impedance. The tool computes matching bias resistor pair (pull-up on A line, pull-down on B line) that biases the idle line at your target V_diff.

Bias resistors are placed at ONE end of the bus only (usually the master). Verify total bus load impedance stays ≥ 54Ω (minimum for 32-node RS-485 drivers). Everything runs client-side.

Frequently Asked Questions

Why bias?

RS-485 transceivers go tri-state on the bus when no driver is active. Without bias, receiver input floats and any common-mode noise registers as data. Bias pulls the line to a known state (typically B-A > 200 mV) during idle.

Fail-safe?

Modern failsafe RS-485 receivers (MAX3485E, SN65HVD75) have internal bias — tens of kΩ pull-ups/pull-downs. External bias only needed for non-failsafe transceivers or very long buses.

Typical values?

500-750Ω bias resistors at one end of the bus, in parallel with the 120Ω termination. Effective termination impedance drops to ~100Ω but receiver sees reliable differential bias.

Common Use Cases

Modbus RTU

500 Ω pull-up to VCC + 500 Ω pull-down to GND + 120 Ω termination at master end.

Theater DMX

DMX512 bus bias: 120Ω termination at last fixture, no explicit bias needed with failsafe transceivers.

Long Industrial

Multi-drop 1 km factory bus: split termination + bias at master, simple 120Ω at far end.

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