I'm trying to figure out whats the purpose of a NOT gate that's connected on pin CB2 of the 6821. In Tom Owad schematic this gate is connected to pin CB2 and then coupled with a capacitor to pin PB7 which is DA. To me this is useless because the cap will filter the output of the gate. On Briel's schematic the output of the gate is directly connected to the DA signal which I'm guessing both CB2 and DA are outputs and may probably create a logic contention. In the R1 plus the gate disapeared and instead there is capacitor connected from CB2 to ground. The original schematic is more similar to Briel's schematic but it shows that CB2 is an output and PB7 is an input which makes more sense.