Modicon made Modbus open and simple, and it became industry's de facto standard

When programmable logic controllers arrived in the late 1970s, automation grew quickly but there was no common way for controllers and instruments from different makers to exchange data. Each vendor had its own proprietary scheme, so connecting devices meant custom engineering.

Modicon, in 1979, chose the opposite path. It released Modbus as a non-proprietary standard and welcomed competitors to implement it. The protocol moved only two kinds of data — registers (16-bit unsigned integers) and coils (single bits) — via a simple master-slave request/response command set.

The restraint fit the hardware. Programmers of the era often had just 64 bytes of RAM, so every byte mattered; a Modbus master could often fit in around 1K of program space. Simple devices like photoelectric sensors could now send their data to a control system over a daisy-chained RS-485 network instead of being hard-wired point to point.

That combination — open, tiny, cheap hardware, easy to understand — made Modbus the first widely accepted fieldbus. Within a short time hundreds of vendors implemented it and it became the de facto standard for industrial communications, an honest case where the simplest option beat the clever ones.