An RS-485 bus that works on the bench and fails in the field usually fails for one of three reasons. The idle line floats because the fail-safe bias was never sized properly, so the receiver sees noise as data the moment every driver goes high impedance. The cable is long enough that reflections corrupt the signal because termination was guessed rather than checked against the bit time. Or a thirty-third device was added to a bus that the TIA-485 32 unit load limit could not carry, and the whole segment degrades. These are not exotic failures. They are the everyday consequences of designing the bias network and the termination by rule of thumb instead of by calculation.
The noIM₃ RS-485 Biasing & Termination Calculator is a single screen workstation for getting the bias network and the termination right before commissioning. The bias solver returns the pull-up (A line to V_CC) and pull-down (B line to GND) resistors needed to hold the idle V_diff above a chosen target — 200 mV spec minimum, 300 mV recommended, 400 mV for PROFIBUS and long runs, 500 mV high margin, or 750 mV extra margin — against the supply rail, the cable characteristic impedance, the node count, the per node unit load, and the termination configuration. Each calculated resistor is returned alongside the nearest E24 standard value so the bill of materials is grounded in real parts rather than computed ohms.
The termination analyser computes the cable propagation delay against the bit time at the configured baud and reports the cable length above which termination is required — the 10% bit time rule, where the cable round trip delay must stay under 10% of the bit time. Both ends termination, far end only, and no termination are all supported, and the configured length is flagged REQUIRED or OPTIONAL against the threshold. The bus load check totals the unit loads against the TIA-485 32 UL limit, with the per node unit load configurable across 1 / ½ / ¼ / ⅛ UL devices, so you can see whether another node fits before the bus must be split. For transceivers with internal failsafe (SN65HVD / MAX3485 family) the external bias network is suppressed and the design is flagged as relying on the IC failsafe.
Protocol presets cover Modbus RTU, DMX-512, PROFIBUS DP, BACnet MS/TP, and DNP3, each applying credible baud, cable, termination, and bias defaults so the design starts from a real baseline. Outputs include the idle bus voltage breakdown (V_A, V_B, V_diff), the margin over the 200 mV TIA-485 spec, the termination threshold length, the bias and termination power dissipation, a node count sensitivity table, a circuit schematic visualisation of the bias and termination network, and a protocol profile reference for cross checking the design against industry deployments. All computation runs entirely in your browser.
