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RS-485, Modbus RTU and Profibus DP networks

Enter protocol, baud rate, cable length and devices to see whether one segment is enough, how many repeaters and terminators to use and how long the Modbus master takes to poll every slave.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

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Modbus RTU polling

Shielded twisted-pair cable with 100 to 130 Ω impedance (Profibus type A: 135 to 165 Ω, 30 pF/m). Daisy-chain topology, no star.

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Maximum segment length versus baud rate

RS-485 from the TIA-485-A practical curve (4,000 ft up to about 90 kbit/s, then L × rate ≤ 10⁸ m·bit/s); Profibus DP from the IEC 61158 table for type A cable.

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Modbus scan time versus baud rate

Same network and messages at each rate. Past a certain point, slave response time dominates and a faster rate barely helps.

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Timing of one Modbus transaction

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Method, assumptions and references

Equations used

RS-485 (TIA-485-A practical curve, 24 AWG cable): Lmax = min(1200 m; 10⁸ ÷ rate)
Profibus DP, type A cable: 9.6 to 93.75 kbit/s → 1200 m; 187.5 → 1000 m; 500 → 400 m; 1500 → 200 m; 3000 to 12,000 → 100 m
Segments s: smallest integer with s ≥ L ÷ Lmax and s · cap − 2(s − 1) ≥ devices (each repeater takes one load on both segments); cap = 32 ÷ UL
Modbus RTU: tc = 11 ÷ rate; t3.5 = 3.5 · tc (1.75 ms above 19,200 bit/s); transaction = n_req · tc + max(t_resp; t3.5) + n_resp · tc + t3.5 + delay
Bytes (address + function + data + CRC): 01/02 → 8 and 5 + ⌈N/8⌉; 03/04 → 8 and 5 + 2N; 05/06 → 8 and 8; 15 → 9 + ⌈N/8⌉ and 8; 16 → 9 + 2N and 8
Scan = transaction × messages per slave × number of slaves

Assumptions and limits

  • Daisy-chain topology with termination at both ends of each segment; short stubs. Shielded twisted-pair cable with suitable impedance.
  • The RS-485 length curve is the TSB-89 practical one for 24 AWG; lower-capacitance cables allow slightly more.
  • Modbus scan with no errors or retries, one master and one port. Slave response time comes from the manual or a measurement; drives and meters usually respond in 5 to 50 ms.

Standards and technical documents

  • TIA-485-A (ANSI/TIA/EIA-485-A-1998): Electrical Characteristics of Generators and Receivers for Use in Balanced Digital Multipoint Systems
  • TIA/EIA TSB-89-A: Application Guidelines for TIA/EIA-485-A
  • Modbus Organization: MODBUS over Serial Line Specification and Implementation Guide V1.02; MODBUS Application Protocol Specification V1.1b3
  • IEC 61158 e IEC 61784-1 (CPF 3): Profibus DP
  • PROFIBUS International: Installation Guideline PROFIBUS DP/FMS (2.112) e PROFIBUS Assembling Guideline (8.022)

U.S. regulations

  • NFPA 70 (NEC) Article 725: Class 1, 2 and 3 remote-control, signaling and power-limited circuits; Article 800: communications circuits
  • NFPA 79: Electrical Standard for Industrial Machinery (control and network wiring)
  • OSHA 29 CFR 1910.147: control of hazardous energy when working on networked machines

Frequently asked questions

What is the maximum length of an RS-485 network?

About 4,000 ft (1,200 m) per segment up to roughly 90 kbit/s. Above that, use the rule length × rate ≤ 10⁸ m·bit/s: at 115,200 bit/s, about 2,850 ft. With repeaters, each segment adds another 4,000 ft.

How many devices fit on an RS-485 bus?

32 unit loads per segment. With 1/8 unit-load transceivers, 256 devices. Modbus allows 247 slave addresses; Profibus DP, 126 stations.

How long does Modbus RTU take to poll 10 slaves?

At 19,200 bit/s, reading 10 registers (8-byte request, 25-byte response) with a 10 ms slave response takes about 31 ms per slave, or 309 ms for 10 slaves.

How long can Profibus DP run at 1.5 Mbit/s?

200 m (656 ft) per segment with type A cable. At 12 Mbit/s it drops to 100 m (328 ft); up to 93.75 kbit/s it reaches 1,200 m (3,937 ft). Each segment takes 32 stations, including repeaters.