[Coco] [Color Computer] CoCo USB Circuit update

[James Diffendaffer]

By changing chips I can implement the circuit with 3 chips, but now it
uses a decoder and a selectable address (I was one gate shy of a 2
chip design).  That allows the address of the USB ports to be moved or
additional devices to be interfaced through the unused logic.

Parts:
13 input NAND
http://upgrade.cntc.ac.kr/data/ttl/74133.html
Hex inverter
http://upgrade.cntc.ac.kr/data/ttl/74004.html
3 to 8 line decoder
http://upgrade.cntc.ac.kr/data/ttl/74138.html

The R/W* line uses an inverter instead of a NAND gate as does the
interrupt signal but the lines go the same places on the buss and USB
chip as they did before.  Data lines still go straight across.

Attach A4-A6, A8-A15 to NAND inputs.
Attach A7 to an inverter and attach it's output to a NAND input.
Attach the E clock to the last NAND input.
Attach the NAND output to the SL811HS nCS. (chip select)
That gives us decoding of the top 12 bits and E clock.

Attach A1-A3 to the inputs on the 3 to 8 decoder (A1-A, A2-B, A3-C).
Attach A0 to A0 on the SL811HS.
Those decode the last 4 bits of the address.

Attach the desired decoder output to an input on the HEX inverter
and attach it's output to the SL811HS nCS.

Decoder outputs and corrisponding addresses (some addresses are
already used so use caution):
0 - FF70-FF71
1 - FF72-FF73
2 - FF74-FF75
3 - FF76-FF77
4 - FF78-FF79
5 - FF7A-FF7B
6 - FF7C-FF7D
7 - FF7E-FF7F

And remember, only one line attached to each input please!  The only
exception is the nCS line on the USB chip.





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