[Coco] Embedded coco

[peak at mail.polarcomm.com]

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