Ward Christensen spent his entire 44-year career as a systems engineer at IBM — but it was his hobbies that earned him a place in history.

In 1977, when Christensen needed to convert a CP/M floppy disk to an audio cassette, he developed a transfer protocol consisting of 128-byte blocks, the sector size used by CP/M floppies. The protocol proved so versatile and reliable for a variety of platforms that it evolved into XMODEM, which became a standard for transferring data files across dial-up modem connections, especially at slower speeds such as 300 baud.

Christensen’s work on XMODEM earned him a sponsorship from the White Sands Missile Range to dial into the ARPANET. But he was frustrated by the organization’s design-by-committee approach, where ideas languished. When Chicago’s Great Blizzard of 1978 left Christensen and his fellow computing enthusiasts stranded in their homes, Christensen called his friend Randy Suess to develop a way for their local hobby computer club to meet virtually. The two collaborated, with Suess providing the hardware and Christensen the software. Within two weeks, the Computerized Bulletin Board System (CBBS) was up and running.

CBBS became the first of tens of thousands of dial-up BBSes that proliferated over the next twenty years. BBSes formed some of the first online communities and became important shareware distribution nodes for early game companies. The groundbreaking innovation earned Christensen multiple awards and recognition, including a 1993 Pioneer Award from the Electronic Frontier Foundation.

Christensen retired from IBM in 2012, after which he remained active in Build-a-Blinkie, a nonprofit that teaches basic computer hardware skills. “I [can] think of no finer testimony to the soul behind this pioneer than the fact that up to the end of his life, he was teaching very young children how to solder together electronics to get them interested in science and engineering,” said Jason Scott, creator of BBS: The Documentary.

Christensen died at home from a heart attack at the age of 78.