RS-485 Bias/Termination Calculator
Calculate bias and termination resistor values for RS-485 networks based on cable impedance and node count.
RS-485
bias
termination
resistor
RS422
Modbus
serial
bus
Input values
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Enter values and click Calculate to see results here.
Formula
R_term = Z₀ (cable characteristic impedance)
R_bias = V_cc / (I_bias × N_nodes)
Typical: R_term = 120Ω, R_bias = 390-560Ω
Worked Example
Given: A 120Ω cable with 8 nodes, 5V supply.
Step 1: Termination = 120Ω (one at each end of the bus)
Step 2: Bias resistors: Pull-up = 560Ω to +5V on A line, Pull-down = 560Ω to GND on B line
Step 3: Total bus loading = 120Ω (termination) + 560Ω (bias) = 97Ω per direction
Check that the RS-485 driver can handle the load. Most drivers support 32 unit loads.
Engineering Notes
- Why termination: Without termination, signal reflections cause data corruption at high baud rates or long cable lengths. Always terminate both ends of the bus.
- Why biasing: When no driver is active (all receivers idle), the bus floats. Bias resistors set the idle state to a known logic high (A > B by 200 mV minimum).
- Cable impedance: Use the cable's characteristic impedance (Z₀), not DC resistance. Common values: 120Ω (STP), 100Ω (Cat5e), 75Ω (coax).
- Node count: RS-485 supports 32 unit loads. With 1/8 load transceivers, up to 256 nodes are possible.
- Cable length: Max 1200 m at low baud rates (< 9600). Reduce length for higher baud rates.
References
- TIA/EIA-485-A — RS-485 standard
- Analog Devices AN-375 — RS-485 biasing