Simulation result
| Calculation | Result | Meaning | |
|---|---|---|---|
| Maximum simulated baudrate | — | Highest standard baudrate passing the model | |
| Maximum devices at target speed | — | Selected cable, stubs and baudrate | |
| Spacing between ModVibes | — | TI advisory minimum | |
| Maximum cable length at target speed | — | Selected devices, stubs and baudrate | |
| Modelled bus capacitance | — | Informational total; expand for calculation | |
| Estimated far-end signal | — | Model estimate, not a measured value | |
| Voltage headroom | — | Above the ±200 mV receiver threshold | |
| Longest valid sampling window | — | At least 80% required; not TI jitter or BER |
Bus topology
Cable length versus baudrate
Cable length versus voltage headroom
What is included
- HELUKABEL 81910 PUR capacitance, resistance and attenuation.
- THVD2410V worst-case drive and receiver loading.
- Two populated TVS devices and 44 Ω series pair per ModVibe.
- Ideal matched end terminations, stubs and RS-485 unit loading.
Hardware assumptions
Directional scope: the source is fixed at one endpoint of the main cable and every ModVibe is treated as a passive load. ModVibe return transmissions from a stub toward receivers in both cable directions are not simulated.
The inspected dual-port Waveshare FT2232H adapter uses an SP485EE line transceiver. It is stronger and faster than the released ModVibe output path, so it is not used to authorize a higher planning rate.
The worst-case source uses the ModVibe THVD2410VDRCR, its 300 ns fast-mode rise/fall limit and the released 2 × 22 Ω protection pair. SLR low selects its 1 Mbit/s mode; SLR high selects 250 kbit/s.
The simulation assumes the main cable is terminated at both endpoints with resistors exactly matching the selected cable impedance. Missing or mismatched termination is outside this calculator's scope.
The source is a 3.0 V / 54 Ω linear proxy anchored to the THVD2410V guaranteed minimum drive of 1.5 V into the worst-case RS-485 load. The independently configurable source series pair is added to that source impedance. This is not an IBIS driver model.
Decision rules
- Green: simulation and TI's cable-only rule pass.
- Amber: limited simulated or TI cable-rule margin.
- Red: a selected topology rule fails; test or redesign.
Simulation method
The browser simulation cascades lossy transmission-line sections and evenly spaced loaded stubs. It reconstructs an alternating-bit waveform from seven odd harmonics. A simulated pass requires 3× receiver-threshold peak voltage and at least 80% of each bit beyond ±200 mV. This sampling- window screen prevents a narrow voltage peak from passing; it is not TI's PRBS jitter measurement or a BER result. The model requires at least one metre of main cable per ModVibe; denser clustered layouts are outside its scope. These are model criteria, not product specifications.
The raw model can exceed TI's length-rate recommendation because it evaluates a deterministic alternating pattern, not PRBS crossing jitter, noise or production tolerances. TI's point-to-point laboratory results also exceed that rule of thumb. The calculator therefore uses simulation only to reduce the TI cable rate; theoretical headroom is ignored. Agreement with TI's published point-to-point trends is a useful model sanity check, but does not prove the loaded ModVibe multidrop extrapolation.
Speed uses TI's empirical RS-485 length × bitrate rule. That rule is cable-only: it is not equivalent to any specific number of ModVibes and does not include node loading. Reported bus capacitance is informational and is not treated as one lumped transmission-line capacitor.
Release boundary
This page is a topology estimator, not a cable qualification certificate. Combinations in the red zone need a representative eye-diagram and error-rate test. The selected node count is evaluated by the simulation; a generic converter node recommendation is not presented as an electrical limit. TI's published cable measurements use different transceivers, cable and point-to-point wiring; they are reference trends rather than direct proof of the ModVibe multidrop topology. The calculator covers only transmissions from a source at one cable endpoint and does not validate the complete half-duplex bus.