Trying to make an Apple 1 sort of replica: MC6821 PIA behavior

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OrionSoft's picture
Joined: Jan 24 2017
Posts: 8

Since I can't get any answer from V.Briel about the Replica 1 computer
I'm trying to make a sort of Replica 1 computer myself, but I'm not an electronic engineer, just a programmer.

I'm trying to setup the MC6821 PIA, and I have a problem with it. (the exact reference is EF68B21P, it's a 2mhz capable 6821)
After configuring the PIA just like the Woz monitor do (Control Register = 0xA7) whenever I input a signal into the CA1 pin, I get no info in the control register (value is always 0x27), and no output on the CA2 pin either.
I'm not using a propeller chip, I'm trying to use this keyboard/video interface
but right now I'm just trying to debug the PIA behavior with an Arduino and some LEDs to watch the input/ouput,
and can't figure out how to get the PIA to receive the ack from CA1 pin Sad

Using the Arduino, I tried to set the CA1 pin permanently, or just send a pulse, but when I read the Control Register after that, I don't have the bit 7 set.
and, about the CA2 pin, I only get a signal on it when I send 2 pulses on CA1 without reading the Control Register in between (no pulse on the E pin)

What would be great is if someone could tell me what are the signal on the CA1/CA2 CB1/CB2 pin on an Apple 1
High or Low, continuous signal or pulse ?

Thank you

Edit: after reviewing the early Replica 1 ATmega source code, I think I have my answer, but testing the same behavior with my arduino doesn't work
I guess I will need to wire the complete computer and see if it works like that


I'm young retro computing geek, and indie game developer on retro consoles.