+--------------------------------+
	!    APPLE-1 SIMPLE I/O BOARD    !
	+--------------------------------+


This boards provide:
- 8 Digital Outputs (TTL level) with LEDs
- 8 Digital Inputs (TTL level)
- CS lines for input & output on the user connector

All the functions can be accessed without additional software:
Simply solder a couple of wires between CS through-holes and 74154 output section:
- CS_INPUT will be the base address to read signals coming from the real world
- CS_OUTPUT will be the base address to write bits to the output buffer (AKA LEDs / the real world)

Read carefully Apple-1 documentation and memory mapping in order to avoid conflicts 
between periperals and potential damages.

From now on it is now assumed that:
- CS_INPUT through-hole is connected to the through-hole "9" (this will give board's input base address = $9000)
- CS_OUTPUT through-hole is connected to the through-hole "A" (this will give board's ouput base address = $A000)

---------------------
DIGITAL CHANNELS READ
---------------------

Lets'assume that, for example, Digital Inputs are connected as follows:
INPUT 1 = Ground
INPUT 2 = 5V
INPUT 3 = Ground
INPUT 4 = Ground
INPUT 5 = Ground
INPUT 6 = 5V
INPUT 7 = 5V
INPUT 8 = 5V


To read the electric/logic levels of the input connector just read any location between $9000 and $9FFF:
9000 {ENTER} --> E2 will appear

HEX value (E2 in this example) will be the representation of the logic level of the channels (1=HIGH=5V, 0=LOW=Ground):
11100010 --> $E2 (226 Decimal)

Output will reflect the electrical status of the inputs at the moment of reading, as stated above.
Please note that PULL-UP resistors are active, so an open/disconnected pin will have value = 1.

In BASIC: Note that $9000 is decimal 36864. This value overflows INTEGER's BASIC signed integer notation (-32767/+32767) therefore
65536 must be subtracted from the value. 36864-65536=-28671.

PRINT PEEK(-28671) {ENTER}
Decimal 226 will appear on the screen (226 decimal = $E2 = 11100010)


----------------------                                    
DIGITAL OUTPUT WRITING
----------------------

Let's assume that we want to set/assert the following bit/signals on to the output connector:

OUTPUT 1 = 5V
OUTPUT 2 = nothing
OUTPUT 3 = nothing
OUTPUT 4 = 5V
OUTPUT 5 = nothing
OUTPUT 6 = nothing
OUTPUT 7 = 5V
OUTPUT 8 = 5V

To compute the right value to write on to the output register/buffer we must convert it to Hexadecimal (and eventually to decimal):
11001001 = $C9 = 201 decimal

Then the calculated value must be written into any address in the selected output range ($A000 to $AFFF in this example):
A000: C9 {ENTER}

The LEDs will light up accordingly.

IN BASIC: Note that $9000 is decimal 40960. This value overflows INTEGER's BASIC signed integer notation (-32767/+32767) therefore
65536 must be subtracted from the value. 40960-65536=-24576.

POKE -24576,201 {ENTER}

The LEDs will light up accordingly.

Note: Chip Select signals have been replicated to the output connector, in case they are needed for some experiment.

---------------------
    KNOWN ISSUES
---------------------

Sometimes a brief "instability" of outputs may appear before they stabilize.
This is a known issue and it will be fixed in the next board revision.
However it is so quick that hopefully will not interfere with your experiments (i.e. it's too fast to trigger a relay).




March 2020
Claudio Parmigiani