▮ Learn Modbus
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Module 10Physical Network & Transport8 min.md

RS-232, RS-422, RS-485

▮ Overview

What you'll learn

Topologies, biasing, termination, and why your bus quietly stops working at node 17.

Sections
11
Labs
1
Quiz
7 Qs
What you'll be able to do
  • Wire a multi-drop RS-485 bus with correct termination and biasing.
  • Pick RS-232 / RS-422 / RS-485 for a given topology.
  • Diagnose 'works at node 5, dies at node 17' bus problems.
Why you'll need this
  • "A bus has 22 nodes, no bias resistors, and one termination. Which symptom appears first?"
Three things people get wrong
  1. 1.
    Terminating one end only
    Fix Terminate near both trunk ends, never on a derivation, and choose the termination network from the official design case plus the cable and device documentation.
  2. 2.
    Skipping fail-safe biasing
    Fix Use one bus-wide polarization network only when the connected devices require it; verify the values against the device and network design.
  3. 3.
    Mixing A/B polarity across vendors
    Fix Different vendors label A/B opposite ways. Trust the signal name (D+/D-) over the letter.
From the field

The 17th drop

Adding one more meter to an existing RS-485 trunk caused every device to start dropping frames. The added stub was 80 cm — long enough to reflect at 38400 baud. Rerouting the cable as a true daisy-chain fixed the problem without changing any device settings.

Cited sources

Primary sources come from protocol and standards publishers. Secondary sources provide supporting tool, vendor, or reference context.

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