Walk into almost any factory, water treatment plant, or power substation and, somewhere underneath the modern dashboards and cloud connections, you will find a protocol older than most of the engineers maintaining it. Modbus was published in 1979, and instead of fading out like most technology from that era, it has become the closest thing industrial IoT has to a universal language.
Where Modbus came from
Modbus was created by Modicon, the company that had built the first programmable logic controller (PLC), the Modicon 084, a decade earlier in 1968. PLCs from different manufacturers could not talk to each other, and even different product lines from the same vendor often used incompatible, proprietary communication schemes. Modicon designed Modbus as a simple, well-documented request-response protocol for its own PLCs to exchange data with sensors, actuators, and other controllers over a serial line.
The decision that mattered more than the protocol's technical design was what Modicon did next: it published the specification openly, with no licensing fee and no requirement to buy Modicon hardware to implement it. In an industry used to proprietary, vendor-locked communication schemes, that openness was unusual, and it is the single biggest reason Modbus is still around 45 years later.
Why an open, simple protocol outlasted its competitors
Modbus works on a master-slave (today more commonly called client-server) model. A controller asks a device for data at a specific register address, or writes a value to one, using a short, fixed set of function codes: read coils, read registers, write single register, write multiple registers, and a handful of others. There is no encryption, no service discovery, no complex handshake. That simplicity meant a manufacturer could implement Modbus support in a sensor, drive, or meter with minimal firmware and minimal engineering cost, and be confident it would talk to control systems from other vendors.
Because the specification was open and free, competing PLC and automation vendors adopted it too, rather than trying to displace it with something proprietary. Once enough of an industry standardizes on a shared protocol, the cost of replacing it stops being about the protocol's technical merits and starts being about the sheer number of installed devices, trained technicians, and existing wiring that would need to change. Modbus reached that critical mass decades ago, in factories, building automation, solar and battery installations, and utility SCADA systems, and it has stayed there since.
Modbus RTU and Modbus TCP
The original Modbus ran over serial links (RS-232 and later RS-485) using a binary framing format called Modbus RTU, which is still common on the factory floor for connecting drives, meters, and I/O modules along a single twisted pair. As Ethernet became standard in industrial environments, Modbus TCP wrapped the same request-response messages in a TCP/IP packet instead of a serial frame, letting the same simple protocol run over the same network as everything else in a plant, including the gateways and edge devices that now forward that data to cloud platforms. A great deal of what looks like "smart factory" or industrial IoT infrastructure today is, underneath, a Modbus TCP gateway translating decades-old register reads into an MQTT topic or a REST API call.
The IoT and embedded lesson
For anyone designing embedded systems or connected devices today, Modbus is a useful case study in protocol design. It did not win because it was the most capable or the most secure option available in 1979 — by modern standards it has neither authentication nor encryption built in, which is exactly why Modbus TCP deployments need to sit behind a properly segmented network or a secure gateway rather than being exposed directly. It won because it was open, simple enough to implement cheaply, and good enough for the job, and because that openness let an entire industry standardize around it instead of fragmenting into incompatible proprietary schemes. Firmware and protocol choices made on a project today, including in Philippines-based manufacturing and process industries adopting industrial IoT retrofits, often outlive the original hardware, the original team, and sometimes the original company, in exactly the way Modbus has.
If you're specifying a communication protocol for a new embedded product, or trying to get an old PLC or sensor talking to a modern IoT platform, get in touch — bridging legacy industrial protocols like Modbus with modern cloud and IoT systems is a core part of the IoT and embedded development work we do at Fluidwire.
