Re: Prototype card and info needed for project

[Rubywand <rubywand@swbell.net>]

Michael Spangler writes ...
> 
> I'm attempting to build a digital I/O card to run on an apple II, using
> either an 8255 or a 6821 chip. An Apple II makes a considerably more
> flexible controller than a Basic Stamp, and it can store its data on
> disk for later transfer to another computer.
> 
> However, I need a prototype card for the A2 slot, and some details
> about the electronics of connecting the 8255 to the bus. The card can
> be memory mapped, as I don't need any ROM routines. The 16 bytes per
> slot between C900 and C100 should be enough. I can see how to decode
> the address lines, but do I use the device select or the slot select
> pins, both, or something else?
> 

     As shown in the attached pic, you can interface to an 8255 using the
Device Select, R/W, RST, and a few address lines. D0-D7 on the 8255 go to the
D0-D7 Slot card lines.  (The addresses shown are for placement in Slot 2.)

     The decoding uses the 8255's separate R and W lines and permits the IC
to be always selected (i.e. Chip Select is always active) in order to enhance
speed. Including A2 in the decoding makes it easier to add a second 8255.



> Does anyone know a source of prototype cards for the Apple II?

     Jameco may still sell Apple II prototype boards. However, the approach
suggested by Patrick is probably the one used by most experimenters.

 
> Does anyone know the details of interfacing this sort of card? Or where
> the info can be found?
 ....

     After Reset, the 8255 is set for all three 8-bit ports to act as inputs.
If you want a configuration which uses one or more ports as outputs, you
write the setup code for the ports to $C0A3 ($C093 for Slot 1). Here are a
few setups with codes:

Code      Setup

$80        A, B, C are all outputs 
$9B        A, B, C are all inputs
$89        A, B are outputs; C is inputs
$92        A, B are inputs; C is outputs


     The Intel 8255 data sheet does a good job of explaining the different
possible configurations.




Rubywand