The encyclopedia · Engineering & Operations · Technical decision · 1979-2020s
Modicon made Modbus open and simple, and it became industry's de facto standard
Modicon released Modbus in 1979 as an open, dead-simple protocol (registers and coils), so hundreds of vendors adopted it and made it the default fieldbus.
Modicon
the move
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.
why it works
- Releasing the protocol openly let rivals build on it rather than fight it
- Only registers and coils means the data model fits almost any device
- A ~1K implementation fit the severely RAM-limited controllers of the time
- RS-485 gave noise immunity and up to 32 nodes on one multi-drop bus
- Non-programmers could grasp coils/registers and the few commands
what transfers
A protocol that is free and trivially small to implement gets adopted by everyone, because adoption costs almost nothing and the vendor who opens it wins the whole market.
what came after
Modbus grew into the most implemented automation protocol of all time, surviving even as Ethernet-based industrial networks appeared. Modbus TCP/IP adapted the same simple model to TCP/IP, and a vast install base of industrial and building-management devices still speaks Modbus over forty years later. Its simplicity is also at the root of a long-standing criticism: it has no built-in security or authentication.
references
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