ModVibe RS‑485 planning calculator
Explore the trade-off between cable length, connected devices and baudrate for the current ModVibe hardware and HELUKABEL 81910 PUR. This calculator provides an engineering indication under the stated assumptions.
Installation
ModVibe does not support rates below 115.2 kbaud. Lower rates remain available for electrical simulation only.
Engineering assumptions
Cable: HELUKABEL 81910 DeviceNet Thin PUR, 1×2×AWG24 + 1×2×AWG22. It is highly flexible for drag chains, resistant to common coolants and lubricants, and has a 70 mm minimum repeated bending radius.
AWG24 data loop: 2 × 90 Ω/km conductor maximum.
Both cable endpoints are terminated with this value.
THVD2410V fast-mode worst-case rise/fall time.
Conservative model value for each 5.0SMDJ30CA.
Current hardware: 2 × 22 Ω. Lowering this only simulates a hardware change. In this endpoint-source model it affects each ModVibe's passive bus loading; ModVibe transmissions are not simulated.
Below 44 Ω changes the released ModVibe protection network. Surge, EFT and EMC performance must be revalidated and the reduced protection must be explicitly accepted by the customer. The TVS remains the primary surge clamp.
Worst-case default: ModVibe 2 × 22 Ω. Use 0 Ω with a 40 ns rise time to explore the inspected SP485EE gateway source.
Simulation result
Conclusion: 115.2 kbaud passes the model and retains the 30% TI cable-envelope margin. 115.2 kbaud is the highest passing standard rate with that margin.
| Calculation | Result | Meaning | |
|---|---|---|---|
| Maximum simulated baudrate | 115.2 k | Highest standard baudrate passing the model | |
| Maximum devices at 115.2 kbaud | 40 | Selected cable, stubs and baudrate | |
| Spacing between ModVibes | 3.1 m | TI advisory minimum: 12.4 m | |
| Maximum cable length at 115.2 kbaud | 511 m | Selected devices, stubs and baudrate | |
| Modelled bus capacitance | 86.9 nF | Informational total; expand for calculation | |
| Estimated far-end signal | 0.88 V | Model estimate, not a measured value | |
| Voltage headroom | 12.9 dB | Above the ±200 mV receiver threshold | |
| Longest valid sampling window | 94% | At least 80% required; not TI jitter or BER |
- Cable attenuation estimate: 0.91 dB.
- Simulation gives an indication; actual margin also depends on layout, grounding, interference and environmental EMI.
- Directional scope: source at one cable endpoint only; ModVibe return transmissions are not simulated.
- The main cable is assumed perfectly terminated at both ends.
- TI's generic first-transition reference is 12.4 m between nodes; the selected 3.1 m spacing can still work after reflected waves settle.
- The planning source uses the ModVibe driver minimum and the configured source series pair.
Check this setup with our team
Ask about the cable, controller or installation constraints. The assumptions below will accompany your question and remain editable.
Review the assumptions to include
ModVibe RS-485 planning calculator, model Revision 1.1. Cable reference: HELUKABEL 81910 PUR. Cable length: 100 m. ModVibes: 32. Stub per device: 1.00 m. Target speed: 115200 baud. Cable capacitance: 39.8 pF/m. Loop resistance: 180 Ω/km. Cable impedance / end terminations: 120 Ω. Driver rise time: 300 ns. TVS capacitance per line: 5000 pF. Receiver input: 50 pF. ModVibe series pair: 44 Ω. Source series pair: 44 Ω. Attenuation at 125 kHz: 9.5 dB/km. Scope: source at one cable endpoint, matched end terminations; ModVibe return transmissions are not simulated. Engineering indication, not a qualified installation.
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 check 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.
Calculation model Revision 1.1 · iQunet BV