Electronics > Projects, Designs, and Technical Stuff

Cloning a Tandy TRS-80 Model 1

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GK:

--- Quote from: German_EE on July 08, 2019, 04:04:25 pm ---Well, just as an example, shift-9 produces a ) and shift-3 gets me a ยง on my German keyboard but on the International US layout shift-9 produces ( and shift-3 is #.

This could get confusing.

--- End quote ---


AFAIK the make and break codes sent by the keyboard are standardized to the characters, not the character positions on the keys.

GK:
Phew! One marathon weekend session and the motherboard is done (pending checking/tweaking).









GK:
Schematics...........









Rick-S:
Wow, beautiful.  Thank you so much for all you do for the community.  You have an amazing amount of patience to do what you do.  I'm in awe  :-+

GK:
No worries  :)

Sat back to review the final design and made a few tweaks and error corrections. That PNP in the power supply labeled 2N3055 should be MJ2955. The TO-3 pass transistor / hybrid regulator for the +5V rail is just a homage to an old-school computer regulator. Aavid-brand TO-3 heatsink part #506007B00000G looks just like the heatsink originally used in the TRS-80 too.

I also deleted the two 74HCT00 NAND gate ICs on page 2, which multiplex the chip-select lines from the address decoder ROM for user-selection of BASIC Level 1 or Level 2 and 16k or 48k of RAM. I can instead achieve the same function just by page addressing the address decoder ROM from the switch inputs instead.

Also added a TVS diode across the contacts of the tape drive motor switching relay.

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