Alphatype shared their proprietary firmware and hardware specs with Knuth, and he did even discuss a few questions about it with some of their engineers, I believe. It was all under a non-disclosure agreement, carefully adhered to; I never saw the documents, even in his office.
The key issue was that their firmware insisted on loading proprietary font outline info from the included 5.25" floppy drives, while we needed to be able to have individual characters downloaded dynamically for caching in the machine's 64K (or less?) of RAM, intermixed with the per-line display list info, such that once you started the stepper motor that controlled the horizontal worm-screw that moved the lens over the paper, everything would be ready as it moved along; having to stop and restart a line would inevitably lead to jitter among the characters, as mechanical systems are subject to hysteresis -- you can never get back to exactly where you (thought you) were. I vaguely recall Knuth grousing that the standard firmware couldn't even be proved to be able to handle an arbitrarily complex line of text, and thus his full rewrite.
Ultimately, the output was not completely satisfactory. Tall parenthesis and integral signs are made up of multiple pieces (as that's how TeX and Metafont handle them), and because of the alignment issues mentioned above, plus the fact that the individual pieces were "flashed" onto photographic paper separately, so where they met there was a bit of multi-exposure that lead to some "spread" of the blackness. You can see this if you look closely at integrals in ACP Vol 2, 2nd Ed.; they're a little chubby right along the centerline.
I mention all this as it's yet another case of throwing away a prototype, this time software plus hardware. The Alphatype was replaced by an Autologic APS Micro-5, which didn't use a moving lens, and was generally more digital, and thus had no problem with characters made up from pieces. And no firmware needed replacing, though we did have to run it in a completely unintended mode where each character was sent in run-length format each time it was typeset (meant for occasional one-off logos); this was wildly inefficient and slow, but as the APS used long rolls of photographic paper rather than single large sheets like the Alphatype, it could run unattended for hours to produce many dozen pages at a go. At least Knuth didn't have to rewrite any firmware for it (nor software; I handled it, and avoided telling him how grossly inefficient it was, as I thought it would bother his soul).
The key issue was that their firmware insisted on loading proprietary font outline info from the included 5.25" floppy drives, while we needed to be able to have individual characters downloaded dynamically for caching in the machine's 64K (or less?) of RAM, intermixed with the per-line display list info, such that once you started the stepper motor that controlled the horizontal worm-screw that moved the lens over the paper, everything would be ready as it moved along; having to stop and restart a line would inevitably lead to jitter among the characters, as mechanical systems are subject to hysteresis -- you can never get back to exactly where you (thought you) were. I vaguely recall Knuth grousing that the standard firmware couldn't even be proved to be able to handle an arbitrarily complex line of text, and thus his full rewrite.
Ultimately, the output was not completely satisfactory. Tall parenthesis and integral signs are made up of multiple pieces (as that's how TeX and Metafont handle them), and because of the alignment issues mentioned above, plus the fact that the individual pieces were "flashed" onto photographic paper separately, so where they met there was a bit of multi-exposure that lead to some "spread" of the blackness. You can see this if you look closely at integrals in ACP Vol 2, 2nd Ed.; they're a little chubby right along the centerline.
I mention all this as it's yet another case of throwing away a prototype, this time software plus hardware. The Alphatype was replaced by an Autologic APS Micro-5, which didn't use a moving lens, and was generally more digital, and thus had no problem with characters made up from pieces. And no firmware needed replacing, though we did have to run it in a completely unintended mode where each character was sent in run-length format each time it was typeset (meant for occasional one-off logos); this was wildly inefficient and slow, but as the APS used long rolls of photographic paper rather than single large sheets like the Alphatype, it could run unattended for hours to produce many dozen pages at a go. At least Knuth didn't have to rewrite any firmware for it (nor software; I handled it, and avoided telling him how grossly inefficient it was, as I thought it would bother his soul).