The legacy of SCADA and serial protocols: Laying the groundwork for MQTT

By Arlen Nipper, Co-creator of MQTT and CTO of Cirrus Link Solutions

In this four-part article series, I’ll explain the technologies and events that led up to Andy Stanford-Clark and me creating MQTT as we know it today.

At the time, I was the CTO of Arcom Control Systems. Arcom developed and manufactured SCADA industrial protocol converters that oil and gas companies used to interface their legacy SCADA equipment with more modern SCADA host software.

The first wave of industrial controllers: Built on serial interfaces

Industrial communications protocols began to emerge in the mid to late 1970s, following the commercial availability of UART ICs—chips that formatted data into a start bit, 7 or 8 data bits, and a stop bit. These UARTs had to be paired with physical drivers to meet industrial requirements for distance and robustness.

The RS-232 specification, already in existence by the late ’70s, quickly became the physical standard. Companies like Digital Equipment Corporation (DEC), Modicon, and IBM started including RS-232 ports on their equipment. Within this context, Gould Modicon introduced one of the earliest industrial Poll/Response protocols over RS-232: Modbus, in 1979.

Despite the introduction of Modbus, it remained proprietary to Modicon for years, and hundreds of other “serial protocols” emerged throughout the ’80s and ’90s. All of them shared one critical constraint:

With serial communication, only one device can talk at a time.

This forced all industrial protocols to adopt a Poll/Response structure. The SCADA host initiated all interaction (Poll), and the remote device responded (Response).

Communicating over 4-wire circuits with modems

It was one thing to connect RS-232 serially to equipment in the same room — but pipeline SCADA networks needed to span hundreds of miles. This is where two key technologies came into play:

1. 4-wire leased circuits from AT&T
2. FSK (Frequency Shift Keying) modems

AT&T would install a dedicated 4-wire circuit to almost any location in the U.S., which allowed central control rooms to connect to remote facilities. These multi-drop modem circuits became the backbone of early SCADA networks. Companies like Phillips 66 used them to poll remote equipment round-robin from a central location.

From the 1970s to the early ’90s, AT&T’s leased circuits were the standard for wide-area SCADA. The services were first-rate and government-subsidized, making them widely accessible.

But in 1984, AT&T was broken up into seven “Baby Bells,” which was the beginning of the end of reliable 4-wire circuits. Over the next decade, prices increased and reliability decreased, pushing customers to look for new solutions.

Enter VSAT — and its protocol problems

The gap left by AT&T’s decline was filled by VSAT (Very Small Aperture Terminal) providers like Gilat, GE Spacenet, Scientific Atlanta, and even AT&T Tridom. These vendors could deliver communication links to remote facilities to replace the existing 4-wire modem technology.

However, VSAT came with heavy baggage. Most vendors required use of additional proprietary protocols to get data over the VSAT infrastructure. Double whammy now! This meant Arcom’s converters now had to deal not only with each device’s protocol, but also with the custom protocol of each VSAT system.

Two layers of complexity — on every message.

This increased cost, slowed down data delivery, and created new maintenance headaches.

Coming up next: how TCP/IP and a single visionary customer opened the door to a brand-new communication model—what would eventually become MQTT.