Re: Jeri board and extra things
Date: 2001-05-03 15:23:20

On Thu, 3 May 2001, Ruud Baltissen wrote:

> Hallo Nate,
> > Use bus transceivers. These are bi-directional buffers of a sort, ...
> You probably mean the 74LS245 or equivalent. Let's assume we use one in the
> databus. First question, when should it be enabled? One possible answer is:
> whenever IO1, IO2, ROML and ROMH are active Low. But then what about the
> REU?

If the transciever set is the first thing out of the cart port, the DMA line
can be used to reverse the direction of the address transcievers.  Problem
fixed ;)

> I have thought about it many times but I never realy needed it. An other
> important point: I had no friends to discuss it with. Now I have :)
> Let's do some brainstorming. I assume the critical point is buffering the
> data- and addressbusses and R/W. Buffering of incoming signals is not needed
> IMHO. Buffering of outgoing signals can be realized by a 541.

The 244 can buffer.. Why are you using a latch?

> We'll use 245's to buffer the addressbus. Output Enable (OE) always (L).
> Direction (DIR) depends on wether DMA is activated or not.

RIght.  and on the data bus, DIR is connected to R/W, and inverted by DMA. 
Hm. this could be a challenge.

> We'll use 245's to buffer the databus as well. Pin 11-18 to the C64, pin 2-9
> to the external world.
> OE should be (L) whenever:
> - IO1, IO2, ROML and ROMH are (L)
> - 6510 performs a write
> - DMA is activated
> DIR should be (H), data external -> C64, whenever:
> - 6510 performs READ
> - when during DMA external device performs WRITE (REU, C64-CP/M-cartridge)
> Direction of R/W depends on DMA or not. Two 125 buffers will do I think.

However, if a cart-port device needs to DMA from/to the I/O region (i.e. the
cart slot), the data bus needs to be disconnected.  So, the game and I/O
lines must disable the transciever so it doesnt step on the data.  (this is
the challenge stated above)  Is this a working solution?

> It seems I covered everything, but I already know one situation where the
> interface would not work: what if somebody wants to read from hardware NOT
> using the I/O-lines but decoded the $DE00-$DFFF area itself? It is just a
> question as IMHO it would be a very unwise thing to do because of
> buscollisions when reading from the RAM under the I/O.

Unless you also tweaked the cart port for a single cycle, triggering ultimax
and disabling the ram/rom underneath ;)

> Feel free to shoot on it. I also would appreciate it if you mentioned the
> fact you found nothing.

This message was sent through the cbm-hackers mailing list.
To unsubscribe: echo unsubscribe | mail

Archive generated by hypermail 2.1.1.