Hello everyone, I have recently found my old Apple IIe computer in my father's garage. That led me down a rabbit hole and I am now trying to understand the schematics of the MMU.
These schematics seem to be official Apple documents and they are dated Feb. 1983 -- after the Apple IIe's release. So, they are probably very close to the final MMU design.These schematics also probably went through multiple iterations and verifications and should be free of obvious errors.
Yet, there is something really weird behind the logic for the evaluation of INH' (located near C-2 of "MMU Logic 2 of 2", middle right). I'm probably missing something, but all the logic behind and up to the component L8 is ignored.
-If 64K is HIGH, H3-6 will be high-impedance and the value of INH'/SC0 (an input on the MMU, pin 15) will be used.
-If 64K is LOW, H3-6 will be the result of all the logic behind, but it doesn't matter since the N4-9 pin of the OR gate will be HIGH.
Truth table for this:
There is another schematic for the MMU, maybe that one could help us understand:
This one is much harder to read, but it is really fantastic to look at -- to think all this was drawn and designed by hand!As a hobbyist, it's difficult to know if my understanding of it is wrong (it most probably is).This is what I understand of it's implementation of INH'
There are two pairs of NOR gates NOR_1 and NOR_2 in the top-middle, and NOR_3 and NOR_4 below. Both NORs in a pair have the same inputs, but the top NOR of the pair seems to be an enable/disable for the other NOR. (I have never seen such a notation on the NOR output before but again, I'm an amateur)
The output of the pair can be HIGH or High-Impedence, and the two pairs operate one the inverse of the other.
Since a JFETs will block current if it's gate is HIGH and allow current if it's gate is High-Impedence, this is what I think Ob will be:
The input of DET.R is connected to both Ob of DET.U and INH'/SC0. I'm not sure exactly what DET.R does but I think it function similar to a pull-down resistor. So IA of DET.U will HIGH if Ob or INH'/SC0 is HIGH.
Merging everything gives this truth table:
We get the same values for INH' in both schematics. That can't be right because in both cases, INH' can be simplified to:
(not 64K) or (64K and INH'/SC0)
We don't need L8!
I'd be grateful if someone could tell me where I'm wrong.