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Cloning a Tandy TRS-80 Model 1

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kizmit99:

--- Quote from: oPossum on January 08, 2019, 06:25:13 pm ---This stuff was easy to find 20 years ago when I wrote that emulator!

--- End quote ---

Thank you!  :-+ :-+ :-+

Yeah, it seems like everyone who used to host stuff like that got copyright scared, because a lot of what seems like it used to be out there isn't anymore...   Thanks for the pointer - exactly what I needed.

GK:
Okay, thanks. I have a copies of both "Level 1 Users Manual" and "Level II BASIC Reference Manual" but have yet to sit down and have a read through.

I just finished laying out the video generator PCB. The hardware configuration will be similar to my PET clone - a separate motherboard and a video generator board. What do you guys think of the crosshatched ground plane as opposed to a solid pour? Gives it that vintage-ish look/feel?

Along the lines of the vintage look/feel theme, on the motherboard, like in the original TRS-80, I've decided to base the +5V rail regulator on a 723 rather than a three-terminal 7805 or equivalent. The pass transistor will be a TO3-packaged 2N3055 PCB-mounted on one of those rectangular, finned heatsinks. It's a pity that the MC3423 OVP/crowbar has gone obsolete. Anyone know of an alternative? Alternatively I can just use a small signal PNP switch and a zener diode to trigger an SCR.



wilfred:
The crosshatched ground plane does look good. Since you're not reproducing the PCB I don't think it is critical. But, I also don't see a need to change it.

You can get the MC3423 from Rockby.

GK:

--- Quote from: wilfred on January 10, 2019, 06:10:57 am ---You can get the MC3423 from Rockby.

--- End quote ---


Yes, but NOS only. I'm not going to use anything that isn't a current production part. I was mostly thinking of the MC3423 from a nostalgic point of view. I don't recall ever seeing a second source for it or any kind of (equally popular) substitute. There are lots of different brown-out/MPU-reset power supply supervisory ICs currently available, but an over voltage protection chip with an output drive intended to fire an SCR not so much. Anyway, I can make a crowbar trigger circuit easy enough with just a few discrete parts. The original TRS80 PSU didn't have any kind of OVP at all by the way. Slip with your DMM probes around the feedback pins of one of the two 723s and you can then go and joyfully tally up all of your fried logic chips.

GK:
Not terribly exciting, but the preliminary first page of the motherboard schematic. The RAM of the original TRS-80 keyboard unit was expandable to was 16k max. If you had an expansion unit, that could hold an additional 32k for 48k in total. I'm going to put the full 48k onto my motherboard. My clone design will be theoretically compatible with an original expansion unit, so long as the 32k of RAM inside the original expansion unit is disabled to prevent it from conflicting with my clones upper 32k RAM block. The easiest way to disable the expansion unit RAM would be to lift the output enable pin on the RAM's read output data bus buffer and wire it to be fixed HI-Z.

It for this reason that I am not recreating the hardware to generate two specific drive signals originally present at the expansion port connector; these signals being MUX and !CAS, which were originally present exclusively for the purpose of addressing and refreshing the MK4116 dynamic RAM ICs used in the original expansion unit. All of the expansion interface control logic is shown in this schematic.

Page 2 will contain the RAM, ROM and address decoding, the header for connecting to the video generator board and bugger all else. Once you strip from the original design all of the logic for refreshing and multiplexing the addressing of the original DRAMs, simply the address decoding logic to a single AT27C256 programmed with a lookup table, use a single SRAM chip for the full complement of RAM and and a single AT27C256 for the ROM, there's not an awful lot to it any more.

Kinda boring, in a way  :)

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