▮ Learn Modbus
·
Module 09Physical Network & Transport8 min.md

Serial Communication Basics

▮ Overview

What you'll learn

Baud, parity, stop bits, timing — the fundamentals every RTU integrator must own.

Sections
11
Labs
1
Quiz
7 Qs
What you'll be able to do
  • Configure baud, parity, and stop bits to match a Modbus RTU server.
  • Calculate the 3.5-character silent interval at a given baud rate.
  • Recognize timing-related failures from a capture.
Why you'll need this
  • "A device responds at 19200,N,2 but your tool defaults to 9600,E,1. What do you change first and why?"
Three things people get wrong
  1. 1.
    Mixing parity and stop-bit defaults
    Fix Default is 8-E-1 (even parity, 1 stop). If no parity, use 8-N-2.
  2. 2.
    Ignoring the 3.5-char silent interval
    Fix At 19200 baud and below, calculate the 1.5- and 3.5-character intervals. Above 19200 baud, the official guide recommends fixed 750 µs and 1.750 ms intervals.
  3. 3.
    Treating timeout as 'wrong address'
    Fix A timeout only proves that no valid response arrived before the client deadline. Check reachability, settings, address or Unit Identifier, framing, timing, gateway routing, and physical evidence.
From the field

Two stop bits, one bad cable

An integrator could read at 9600 but timed out at 38400. The cable had marginal capacitance — fine at low baud, sloppy at high. Adding a second stop bit hid the symptom for a week until traffic doubled. The fix was the cable, not the settings.

Cited sources

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

This module is paginated — step through the sections, run the labs, then take the quiz. Progress is saved locally.

Print one-pager →
Email feedback