Electronics > Projects, Designs, and Technical Stuff

Cloning a Tandy TRS-80 Model 1

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kizmit99:
I'm pretty sure the boot code will treat either 0x00 or 0xFF (on a read of the FDC Status register) as no FDC present. 
The original chips used in the design would pretty consistently read a floating databus as all ones. 
Your CMOS chips likely aren't quite as predictable.

GK:

--- Quote from: GK on January 26, 2019, 03:30:33 pm ---Just a random brain fart / stab in the dark here - I wonder if the memory devices used in the original TRS-80, paralleled up on the data bus, still weakly pulled down their data I/O pins when de-selected?

--- End quote ---


BINGO!

I just bunged a 4k7 pull-down resistor on each line of the data bus and voila! Boots without BREAK every time!

GK:

--- Quote from: kizmit99 on January 26, 2019, 03:46:38 pm ---I'm pretty sure the boot code will treat either 0x00 or 0xFF (on a read of the FDC Status register) as no FDC present. 
--- End quote ---


Well, I can confirm 0x00  ;)

kizmit99:
I'm using 0xff and that also works...  Plus, I was walking through the boot code several weeks back, and remember that code, as it was reading the register, then adding 1, then comparing to 2.  Seemed like an odd thing to do until I realized 0xff was -1, so +1 and now your checking for 0 or 1 (instead of -1 and 0).  Just an odd bit of hoops to reduce the code size (typical assembly level trickery)...

GK:
I think passively pulling up to 0xFF will be the preferable way to go in the final design, simply to minimize loading on the bus. For example the 74LS367 line drivers used in the original expansion unit could sink 24mA, but only source 2.4mA while complying with TTL threshold levels.

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