Apple 1 Replica What's the purpose of this NOT gate on CB2?

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Joined: Jan 29 2016
Posts: 4

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.

Apple 1 NOT gate.jpg238.38 KB

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speedyG's picture
Joined: Nov 16 2011
Posts: 2421
Re: Apple 1 Replica What's the purpose of this NOT gate on CB2?

Hello reb2015,

welcome to the board....
the signal at CB2 is used to control the B Port and the gate causes a delay....
in general such a delay is used to delay the time when the chip will accept
the data at Port B to contain valid Data.



In early days I had a lot of money but no time - now I have no money but a lot of time....
the second part includes less friends but a lot more joy on life....

resman's picture
Joined: Feb 9 2006
Posts: 214
Re: Apple 1 Replica What's the purpose of this NOT gate on CB2?

It might also help to understand how port B is being used on the Apple 1. It is connected to the display logic and is used to handshake the data being output. From the Apple 1 ROM:

<br />
ECHO    BIT DSP<br />
        BMI ECHO<br />
        STA DSP<br />
        RTS<br />

The data port is being used as both input on bit 7 (terminal output ready to accept data) and the character data on bits 0-6.

NOTE: this code is backwards from section III of the Apple 1 Operations Manual (BPL vs BMI).

It could be that Woz didn't have this NOT gate there originally to bit 7. That might explain why there is a discrepancy between the actual ROM code and the description in the manual. It doesn't appear there is an logical benefit to inverting the signal on bit 7 of the data port (DA to the terminal appears to want it inverted). Maybe it was one of those "Woz-isms". And adding the cap could also delay/clean up the reading of the the DA value.