Tuesday, November 3, 2015 [Tweets] [Favorites]

Chuck Forsberg, RIP

Crown Memorial Centers (via Jason Scott, comments):

In 1974, Chuck was recruited by Sidereal Corporation, a small startup computer communications company. When he started, Chuck did Sidereal’s engineering work on the dining room table of his houseboat. He was both the hardware and software engineer for Sidereal’s first project, the Micronet.

In the early 1980s, when Chuck was designing specialized word processing hardware at a company called CDI, he made computer programming history. In his free time he wrote a file transfer protocol software that would change his career, and propel him into an elite group of computer software pioneers.

Initially he released YModem, a program that improved on another programmer’s work called XModem. Chuck’s notoriety grew and a large timeshare company, needing an even better data communications program, contracted with Chuck to write an improved version.

The result was ZModem. ZModem was good, really good, and Chuck became more prominent in the industry. At the time, noted PC Magazine columnist John Dvorak said about Chuck’s work: “Here’s the simple fact. Zmodem is the state of the art protocol for microcomputers. … It’s fast and bullet-proof.”


The widely adopted ZMODEM used a sliding window protocol. Rather than wait for positive acknowledgment after each block is sent, it sent blocks in rapid succession and resent unacknowledged blocks later. By avoiding delays due to latency, the bandwidth usable for transmission more closely approached the bandwidth of the underlying link. ZMODEM could also resume interrupted transfers without retransmitting the already-received blocks.

Update (2015-11-03): Wolf Rentzsch:

Sadness, ZMODEM was a great protocol. Implementing it on Classic Mac OS lead me to create Red Shed Threads


Stay up-to-date by subscribing to the Comments RSS Feed for this post.

Leave a Comment