Net+ N10-009 §2.1 Lane B · Industrial OT FREE

Modbus RTU — serial master/slave byte exchange

Modbus RTU is the 1979 serial protocol that still moves bytes between PLCs, RTUs, and field sensors in every data-center BMS you've touched. This page renders the wire-level byte exchange so you can see what a request frame, turnaround silent interval, and response frame look like at 9600 baud — and what changes when you push the baud rate, payload, or line noise.

Animation

Engineering pitfalls

Silent-interval violation breaks frame alignment

If your master polls faster than 3.5 character times between frames, slaves can't tell where one frame ends and the next begins. At 9600 baud, one character time = 1/960 s ≈ 1.04 ms; 3.5 char times ≈ 3.65 ms. Many PLC defaults sit at 1 ms polling and silently miss responses.

Termination resistor missing → reflections corrupt CRC

RS-485 needs 120 Ω termination at both ends of the bus. Skip it and the wave reflects, the CRC fails, and you'll watch your reliability counter climb without ever seeing a hardware fault. The page's line noise slider simulates this — push to 5% and watch frames start to die.

Ground loop between master and slave

Modbus RTU runs differential, but the signal ground still matters. A 1 V common-mode shift between master and slave grounds will throw bytes into the noise floor. Always tie signal grounds across the bus; never rely on chassis-to-chassis paths.

Driver fan-out limit (32 nodes per segment)

Standard RS-485 transceivers drive 32 unit loads. Past 32 devices, drop the bias voltage and frames get truncated. Modern 1/4-load and 1/8-load chips raise this to 128 or 256 — verify your transceiver datasheet before exceeding 32.

References

Primary sources
  • Modbus Organization, MODBUS over Serial Line — Specification and Implementation Guide V1.02 (2006).
  • TIA-485-A, Electrical Characteristics of Generators and Receivers for Use in Balanced Digital Multipoint Systems.
  • CompTIA Network+ N10-009 Exam Objectives, §2.1 (Common Networking Cables, Protocols, and Use Cases).
  • NEMA ICS 1.1, Safety Guidelines for Programmable Logic Controllers.