Also in one of the notebook pages attached to that transcript is discussion of the Econet NMI circuit (giving a disableable NMI driven from what's supposed to be an IRQ output on the 6854). The circuit from that page got into the iss1/2/3 BBC, but was ultimately found to be unsatisfactory when the time came to write the software to drive it, with a revised version on iss4 (and all early issue boards needing wire-adds to implement the new version).
Original version used DTR out of the 6854 to do the interrupt disable; I think that was too slow to access given the banked arrangement of the 6854 registers (and the need to write twice to toggle it on/off), with the revised version re-using the address decode for the station ID links to give a simple read from a fixed address to cause the disable (symmetric with the enable that already worked that way).
(this is the last set of images under "March 1981", page 26 of the PDF, labelled slide 42 in the text)
Original version used DTR out of the 6854 to do the interrupt disable; I think that was too slow to access given the banked arrangement of the 6854 registers (and the need to write twice to toggle it on/off), with the revised version re-using the address decode for the station ID links to give a simple read from a fixed address to cause the disable (symmetric with the enable that already worked that way).
(this is the last set of images under "March 1981", page 26 of the PDF, labelled slide 42 in the text)
Statistics: Posted by arg — Sat Dec 07, 2024 3:48 pm