Don Lancaster created a 6502 version of the TV Typewriter in the late 1970s. It was an example of what, at the time, was called a “glass TTY.” Both the Apple I and Apple II were, among other things, glass TTYs. Another thing that they have in common is that they were all based on MOS Technology's 6502.

The 6502 TV Typewriter was extremely ingenious but showed a tremendous lack of understanding about the aims and purposes of embedded development. The ingenious part of the design was the way Don Lancaster ‘tricked’ the microProcessor (µP) into acting as the CRT Controller (at a saving of some $s and complexity). Basically, a TV sync generator generated an interrupt that dropped the 6502 into a routine that emitted a series of NOPs starting at a convenient address. Those addresses were used to access display memory.

Those of you familiar with CRT timing might see the drawback to the scheme. You have just thrown away ~85% of the µP's bandwidth. The only time left for processing is during “blanking.” All of which makes this a typical Don Lancaster scheme - extremely ingenious and ultimately misguided.

As another example, he was a proponent, admittedly not the only one, of never ever applying for a patent but instead we (those of us in electronics) should rely on “potting” our circuits.

TV_Typewriter