Brian, Awesome reverse engineering! Keep us posted on the progress. This is interesting.
As far as the 74161 counters, the counters can be preloaded by taking the not load pin (pin 9) low. In your schematic it is tied high so no parallel load so it doesn't matter what they are. It would seem tidier to tie them to something and in the early days of CMOS we were told that open inputs make them vulnerable to ESD. I'd have to stare at it a bit longer to tell you what the counters actually do. (Not totally sure if this extended staring will happen,) 😊 Good luck. Lloyd -----Original Message----- From: M100 <[email protected]> On Behalf Of Brian K. White Sent: Saturday, August 19, 2023 11:58 PM To: [email protected] Subject: [M100] NODE DATAPAC Don't ask me why, but I think I've replicated the NODE DATAPAC in KiCAD. https://github.com/bkw777/NODE_DATAPAC https://raw.githubusercontent.com/bkw777/NODE_DATAPAC/db347506f5eeb9908348710727fa3f3b53bfd047/PCB/NODE_DATAPAC_128K_256K.svg This is from beeping out the connections of one with a meter and shining a bright light through the pcb to try to see under the chips. The schematic is not verified, I haven't actually built one, and I didn't desolder anything to uncover the the board to see all the traces, so it might not be complete. I may actually build one just to verify if the schematic is really complete, and also I'm dying to test my guess about just adding piggyback chips to go from 128 to 256k without needing anything else, but I don't want to hack on the original units except the unavoidable removing the old battery and cleaning up the residue. No problem on a new replica board. But can someone explain the theory of operation from the parts & connections? I can see that AD5,6,7 are used to select one of the 8 chips, but I don't understand what A8, A9, /A, & Y0 are actually doing with those counters, nor why they all have all their data inputs NC and not even grounded. Also what is the likely purpose of that user bodge diode on RAM_RST? Only one unit has that, and they both work the same, outwardly. I assume it will have something to do with some weird issue that only happens in some particular situation like you have a printer connected and the computer turned off or something like that. If I make an alternate updated pcb with a removable battery connector or non-recharging coin cell or something, should I add that? -- bkw
