RS-485 / Modbus RTU Bus Design

Will your online instruments actually talk to the PLC at the end of that trunk? Length-vs-baud field limits, propagation-delay check, daisy-chain & stub rules, 120 Ω termination, unit-load count — and IEC 61643-21 surge protection with LW-10 placement for thunderstorm sites. Built for Southeast-Asian plants.

1Bus parameters

Baud rate^[4]

Trunk length L^[3] (10–2000)m

Nodes on segment^[1][2] (2–64)count

Electrical noise^[4]

Cable^[4][6]

Worst stub length^[4][5] (0–10)m

2Site & surge

Lightning exposure^[7][8]

Outdoor exposed run^[8] m

Bus crosses buildings / separate earth points (verify common mode)

Topology is a star / T-branch (not daisy chain)

Results — will the bus talk?

Field limit @baud

—m

Verdict

—

Round-trip / bit

—bit

Check	Value	Limit / rule	Status

Baud	Nominal	Quiet field	Noisy field	Your run

Surge & instrumentation bill

Item	Qty	Note

Quote this bus's instruments at factory price

RS-485 analysers and LW-10 signal surge protectors at the manufacturer's ex-works price — our margin comes solely from the export tax rebate. Termination and 120 Ω STP cable are bought locally; this sheet gives the numbers behind that purchase.

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Sources & parameter basis

[1]	TIA/EIA-485-A — 32 unit loads, 1200 m, common-mode −7…+12 V.
[2]	TI SLLA272 — unit-load definition; 1/8-UL transceivers → 256 nodes.
[3]	Modbus over Serial Line Specification — nominal 1200 m, daisy chain.
[4]	Industrial Monitor Direct Modbus RS-485 guide — field length-vs-baud table (halving per doubling), stub <1 m, termination & bias.
[5]	Innovatic RS-485 engineering note — length ≈ 1200/(1+(kBaud/250)²)^½; stub capacitance 300/kBaud nF.
[6]	Modbus RTU wiring guide — Belden 9841-class cable, bias 560–1000 Ω.
[7]	THOR TRSS-485 / CITEL DLC signal SPD datasheets — IEC 61643-21, classes D1/C2.
[8]	RS485 SPD installation & selection guide — Uc matching, In ≥5 kA, PE lead <0.5 m, SPD/shield earth separation.

Full derivation and worked examples: RS485-Bus-Design-Calculation-Sheet.md (Unit 2 of the I&C manual).

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