[Coco] Embedded coco

[jdaggett at gate.net]

Eric 

Unless something is not clear in the explanation or this is something I got see. 

I understand the 8 input nand gate. The solves the Boolean equation for decoding 
$FF00 to $FFFF.

I can see using a LS138 and A15,  A14, and A13 lines to decode each 8K boundary. 
Again the Boolean equations are satisfied. 

The rest does not make sense to me. By my knowledge a 3 wide 2 to 1 multiplexor 
requires a minimum of 12 gates. (6) 2 input and gates, (3) 2 input or gates and (3) 
invertors. Unless you have more circuitry than what you are saying I have got to see 
how (4) 2 input nand gates and (4) ivertors are switching thre lines.

james


On 21 Dec 2003 at 14:54, peak at mail.polarcomm.com wrote:

> James
> I have completed an address decoder circuit in TTL. It did 
> not take me as long as I thought it would and it was not as 
> difficult as I thought it would be. Here is a verbal outlay:
> 
> First an FFXX detector (one 74sl30 8-input nand gat). Why 
> will be seen.
> Next is a 3-bit address line switcher. It switches Either 
> (A15,A14,and A13) or (A7,A6,and A5) to pins 3,2,and 1 of a 
> 74ls138 chip or it's equivalent depending on the output of 
> the FFXX detector. This switcher requires 4 each 2-input nand 
> gates and 4 each inverters.
> 
> Next is the actual 138 or it's equivalent. I "stole" the 
> gates only equivalent of the LS138 chip from my copy of Radio 
> Shack's coco3 service manual page 98 at the top of the page.
> 
> Here is how this thing works .At the output pins of LS138:
> 
> y7 = ROM not @ $E000-FFFF or = RomVector not @ $FFE0-FFFF
> y6 = CTS not @ $C000-DFFF or = SAM not       @ $FFC0-FFDF
> y5 = BAS not @ $A000-BFFF or = GIME MMU/PALL @ $FFA0-FFBF
> y4 = EBASnot @ $8000-9FFF or = GIME VID not  @ $FF80-FF9F
> y3 = RAM not @ $6000-7FFF or = MPI Soft not  @ $FF60-FF7F
> y2 = Ram not @ $4000-5FFF or = SCS not       @ $FF40-FF5F
> y1 = Ram not @ $2000-3FFF or = PIA2 not      @ $FF20-FF3F
> y0 = Ram not @ $0000-1FFF or = PIA1 not      @ $FF00-FF1F
> Now Just Because I called these enable signal names like "RAM 
> not" does not mean that you have to use ram at that location. 
> You could use the y1 enable signal for a 2716 Eprom for 
> instance.
> 
> BTW how do I Email you the actual schematic of this decoder 
> to you? I am using Polar Communications Email system and I 
> dont know how to do attachments!
> Happy Holidays
> Eric
> 
> 
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