[CoCo] CoCo Hardware Questions

[Fedor Steeman]

James, Alex, Robert, thanks for all the explaining so far! Apparantly the matter
is so complex that there seem to be some disagreements on what exactly is going
on inside the CoCo.

My aim is to create an accurate design model of the processes going inside the
CoCo. This is to better my and other's understanding of the machine, and to have
a basis to implement this model into an object-oriented language to create
emulators and the like. I wanted to start out with the CoCo1/2, move on to the
CoCo3, and eventually even try out several proposals for a CoCo4!

Now, to return to the CoCo1/2, I have changed the Use Case for standard
operations on the basis of the information you all supplied. Please feel free to
edit and correct this use case! Note: I am more interested in the communication
between components and its timing, than in the internal workings of the
different components themselves.

1. VDG gets byte from memory address accessed by SAM
2. SAM sends a high Q-signal to the CPU
3. CPU program counter register is incremented
4. SAM accesses videoRAM at the address specified by... ?some internal register
5. VDG gets byte at RAM address and sends data on to video signal generator
6. VDG increments ?some internal register in SAM
7. SAM sends a high E-signal to 6809
8. 6809 accesses RAM at the address specified by the Program Counter register
9. RAM address sends byte to 6809 (6809 reads RAM address byte)
10.SAM sets Q-signal low
11.6809 acts on byte (executes instruction)
12.SAM sends a low E-signal (to 6809)
13.repeat steps 1...12 until power is cut

Thanks in advance for any help!

Cheers,

Fedor