In the late 1940s—when computer engineers were grappling with unreliable and noisy computer hardware—a group of engineers in a humble lab at the University of Manchester, England, faced a problem so critical that it threatened the computer itself. Machines could produce fragments, but they could not reliably read them.
The consistent study of memory data did not present itself as a major theoretical challenge. It seemed like something familiar: inconsistent computer results.
Engineers including Frederick C. Williams, Tom Kilburn, and GE (Tommy) Thomas traced their failures not to logical errors but to the physical behavior of the machines themselves. The team devised a way to keep the transmitter and receiver synchronized without relying on a separate clock signal. Their innovation, known as Manchester Code or phase codingwhich is coded for each segment with a change over a small period of time, effectively embedding the time information directly into the data stream to become a self-contained signal. Therefore, even if the signal is degraded or the time drifts slightly, the receiver can continue to keep the time based on that standard deviation.
By eliminating the need for separate clocks and reducing synchronization errors, Manchester code makes data transmission more robust across wires and circuits.
Those qualities later made it a natural fit for technologies like Ethernet and early data storage systems. Automation helped standardize the way machines communicate, and laid the foundation for modern digital communication systems.
On 13 April 2026, this achievement was honored with an IEEE Milestone plaque during a ceremony at the University of Manchester. Dignitaries from IEEE and the university attended the event.
It embeds time in signals
Those 1940s engineers at the University of Manchester were working on the systems that went into the Manchester Mark I, one of the first working machines of a stored program.
When problems arose, they used oscilloscopes to check the signals. They discovered that the electrical pipes did not arrive on time. Memory signals also fade over time, making them difficult to read, and if long runs of identical bits occur, the waveform becomes flat without change.
That led to an important realization: The problem wasn’t just determining whether the signal was high or low; the system also lost track of when to sample the signal. Without reliable time signatures, even correctly constructed symbols are poorly read. Bits can be lost or miscalculated because the system is out of sync.
At first, developers tried to fix the hardware. They experimented with stabilizing circuits and generating a constant pulse, trying to impose a regular rhythm on an inherently unstable system. But the repair proved to be fragile, and the electronics of the time could not maintain the required accuracy. So the Manchester team took a different approach.
If the hardware cannot provide a reliable clock, the signal itself must carry one. Instead of representing data as static levels, each bit changes state, with a confirmed change in the middle.
Time embedding in the signal reduces the ambiguous behavior. Machines were suddenly able to reliably transmit, store, and read data—an important step toward an efficient computer system.
Making mysterious signals
The Manchester code faced several problems at once. A common switch allowed continuous time recovery. Transitions seem easier with static levels, and long runs of the same beats no longer produce flat, fuzzy waves. Rather than fighting the imperfections of early electronics, the design worked with them.
From lab curiosity to global scale
What started as a local solution in Manchester shaped digital communication systems for decades, including the first Ethernet technology, where time and shared media were the main challenges.
According to Robert Metcalfe, a member of the team that developed the first Ethernet system at Xerox PARC in 1973, he and his colleagues relied on the Manchester code.
“The Manchester code has solved an important problem: time,” Metcalfe said, explaining that each component carries its own clock and removes the need for a globally synchronized signal.
That self-contained structure wasn’t the only advantage the encoding system offered. In shared coaxial cable, Manchester coding does more than just assign time. Each transceiver left the medium untouched—effectively “turned off”—most of the time, allowing packets from other devices to pass through without interference. Even during transmission, the station was calling the signal about half of the time, leaving the line idle during half of each cycle.
This difference—between a driven signal and an undriven line, rather than 1s and 0s—allowed receivers to return both data and clock time while monitoring the cable for other activity. If the transceiver received a signal when it expected the line to be idle, the signal indicated that another station was transmitting at the same time. In other words, the system can detect conflicts in real time and respond accordingly.
The concept has proven robust beyond local networks. The Manchester code is used on the Voyager spacecraft, now traveling in interstellar space—underscoring its reliability in extreme environments.
Code has also found its way into consumer electronics every day. Infrared remote controls for televisions and audio equipment often rely on the Manchester code using protocols such as RC-5, developed by Philips in the early 1980s. The protocol encodes commands as timed infrared signals transmitted by the handset’s integrated circuit and LED, allowing devices to reliably interpret key presses even with noise and signal distortion. Manufacturers across Europe—and many in the United States—adopted this approach, extending the Manchester code at home.
Why Milestone is important
The IEEE Milestone designation recognizes technologies with lasting impact. The Manchester code is relevant because it solved the fundamental timing problem at a critical time in the history of computing.
Without a way to embed time into the data itself, early digital systems would have remained fragile and unreliable. The Manchester code helped turn them into reliable machines, and made much of today’s digital communication possible.
“The Manchester Code has solved an important problem: time,” -Robert Metcalfe, inventor of Ethernet
Keynote participants at the plaque dedication ceremony included Tom Coughlin, IEEE’s 2024 president; Duncan Ivison, University of Manchester president and vice-chancellor, and Nagham Saeed, chair of the IEEE UK and Ireland Section.
Discussions by Kees Schouhamer Immink (a 2017 IEEE Honor Award winner perhaps best known for his work enabling compact discs and other digital media) and Peter Green (Manchester’s deputy dean of engineering) highlight the code’s lasting impact on digital data storage and communication.
The IEEE Milestone plaque for the Manchester code reads:
“With at this site in 1948–1949, the Manchester code was developed to reliably encode digital data stored on the magnetic tape of the Manchester Mark I computer. It became the standard for computer magnetic tapes and floppy disks and was used in digital communications, including the Voyager 1 and 2 spacecraft and the first Ethernet networks. It has found widespread use in home remote controls, radio frequency identification (RFID) tags, and many network control standards..”
Administered by the IEEE History Center and supported by donors, the Milestone program recognizes outstanding technological developments around the world. The IEEE UK and Ireland Section sponsored the appointment.
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