RS-485 Termination & Bias Resistor Calculator
From node count and unit loads, compute the parallel termination, fail-safe bias resistors, bias current and the maximum line length for the bitrate — the first thing to check when an idle bus drifts or frames go missing.
Key points
RS-485 Termination & Bias Resistor Calculator: From node count and unit loads, compute the parallel termination, fail-safe bias resistors, bias current and the maximum line length for the bitrate — the first thing to check when an idle bus drifts or frames go missing. Formula: V_AB = Vcc·Rp/(2Rb + Rp) ⇒ Rb = (Vcc·Rp/V_AB − Rp)/2. Enter parameters on the DD-CAT IDE online tool page to get results instantly, with a calculation-model diagram that updates with the inputs. Bias resistors only matter when the bus is idle (all drivers high-Z); too small a value makes drivers source tens of mA continuously. Terminations belong at the physical ends; stubs longer than 0.3 m cause reflections — fix the topology rather than enlarging the terminators.
Inputs
V_AB = Vcc·Rp/(2Rb + Rp) ⇒ Rb = (Vcc·Rp/V_AB − Rp)/2Calculation model (updates with parameters)
| Parallel termination Rp | 57.7 Ω | Two 120Ω ends in parallel give 60Ω, further loaded by the nodes |
| Bus load | 8.00 UL | RS-485 drivers are rated for 32 UL |
| Bias resistor per side Rb | 692.3 Ω | One from A to Vcc, one from B to GND |
| Static bias current | 3.467 mA | |
| Bias divider dissipation | 17.33 mW | Present whenever the bus is idle — count it in battery designs |
| Bias strength | Low | Rb < 390Ω is typical; below 200Ω needs a driver-capability check |
| Rule-of-thumb max length at this bitrate | 1041 m | Rule of thumb L ≈ 1.2×10⁸ / bitrate, capped at 1200 m |
Engineering notes
- Bias resistors only matter when the bus is idle (all drivers high-Z); too small a value makes drivers source tens of mA continuously.
- Terminations belong at the physical ends; stubs longer than 0.3 m cause reflections — fix the topology rather than enlarging the terminators.
FAQ
- What is RS-485 Termination & Bias Resistor Calculator for?
- From node count and unit loads, compute the parallel termination, fail-safe bias resistors, bias current and the maximum line length for the bitrate — the first thing to check when an idle bus drifts or frames go missing.
- What is the formula for RS-485 Termination & Bias Resistor Calculator?
- Core formula: V_AB = Vcc·Rp/(2Rb + Rp) ⇒ Rb = (Vcc·Rp/V_AB − Rp)/2. The tool returns results instantly using this formula and the selected parameters.
- What engineering notes should I keep in mind when using RS-485 Termination & Bias Resistor Calculator?
- Bias resistors only matter when the bus is idle (all drivers high-Z); too small a value makes drivers source tens of mA continuously. Terminations belong at the physical ends; stubs longer than 0.3 m cause reflections — fix the topology rather than enlarging the terminators.