01 — WHAT IS MODBUS?

A 1979 protocol for one machine to ask another for numbers.

Estimated 6 minutes.

A 1979 protocol that still runs the plant floor.

Modbus was published by Modicon in 1979 so one machine could ask another machine for numbers. Forty-five years later it is still the most common protocol in factories — because the rules are tiny and almost every PLC ships with it.

Two devices, one question.

One side asks. The other side answers. That is the whole protocol.

It is a conversation, not a network.

Modbus does not define the wires. It defines what one device says to another. That conversation runs on top of whatever transport you have — serial in 1979, TCP/IP today.

Modbus TCP rides on TCP/IP.

Same Modbus message, wrapped in TCP segments, addressed to port 502.

Phone call with a slave.

Master, meet slave. The slave is a small pump station — tank, valve, heater. Pick an English phrase to say; the master frames it as a Modbus request. Reads return data; writes change the slave in front of you. Try all four phrases.

Two roles. One direction.

A Modbus device is either a master or a slave (some vendors say "client" and "server" — same thing). Masters ask. Slaves answer. Slaves never start a conversation; if no one asks, they say nothing.

How many masters?

How many masters can talk to a single slave?

  • Exactly one
  • Many, but only one at a time
  • Unlimited, in parallel

Every exchange has two parts.

There is no streaming and no push. The master sends a request, the slave sends a response, then the line is silent again. If you want fresh data, you ask again.

What if the slave never answers?

You send a request. No response comes back. What happens next?

  • The protocol automatically retries
  • The master times out and decides what to do
  • The slave will eventually respond — just wait

You can explain Modbus in one sentence.

  • A 1979 protocol where a master asks and a slave answers.
  • No streaming, no push — just request and response.
  • Modbus TCP is the same protocol wrapped in TCP/IP, port 502.

Lesson 01 complete.

  • You can describe what Modbus is and is not.
  • You can name both roles and explain why slaves stay quiet.
  • You know what happens when a request gets lost.