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TRS-80 : Getting to "Level III"

G

Great Hierophant

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I was gifted a TRS-80 Model 1 with Level II BASIC and 16KiB of RAM on Thursday. It came with the Video Display (no tint, B&W) and its Power Supply and bunch of manuals and a pair of cassette packages. The system works perfectly, is in excellent shape and has a nice Alps keyboard with all good keys. The display also works, although the contrast dial is a bit strange in its behavior. I have been able to load software in the .cas format and hear it too using my own homebrewed cable I made originally for my IBM PC's cassette port.

There is only so far you can go with Level II and 16K, but its a lot further than Level I and 4K. The system did not include an Expansion Interface, so I am limited to cassette tapes and BASIC programs which can fit into 16K. I'd like to explore the "next level," namely floppy disks or disk images, without spending a fortune. You cannot play Zork from cassette or those Donkey Kong and Pac-Man conversions.

I think I could use my PCs to write disks using ImageDisk, but I am not sure if a 360KiB PC drive will work attached to the Expansion Interface. Maybe a no-twist cable? Would a Gotek with FlashFloppy work? Gotek with HxC firmware?

Is there another option that would be less expensive than trying to find a 2nd hand Expansion Unit? I know of FreHD and MIRE, but that seems to be about the universe of some kind of disk-less solutions for the Model 1.
 
First thing to do is Upgrade the Keyboard Unit with 64K RAM's. Then if you do not have the
Lower Case Modification you might want to do that. Check your Character Generator and
see what Number is on the Character Generator. Might be an easy way to get Lower Case.

PM with your email address and I'll send you documentation on installing 64K in keyboard.

Larry
 
Note that FreHD + Quinnterface will give you an extra 32K RAM. All it lacks is floppy emulation but most anything will run.

The cheapest option is a TRS-IO-M1. It does all the FreHD does except that the hard drive image is read over WiFi (from an SMB drive) instead of from an SD card. Super convenient for cross-development. While you'll likely have to build it yourself it is pretty simple. Though if you really need it pre-built there are a couple people I know who've done so on an ad-hoc basis.

https://github.com/apuder/TRS-IO
 
There is a a small PCB stuck onto the solder side of the main PCB with some yellow foam. It appears to be some kind of bodge as there are two chips on it and a bunch of wires connecting to various points on the main PCB. The chips are an MC14001 and an MC14040. Was this an official bodge or something done afterward by a modder?

My character generator ROM is marked 8046673. I believe it is lower-case capable, not sure whether it supports descenders. I only have seven video S-RAM chips on my mainboard.

Some Level IIs, including mine, say RADIO SHACK LEVEL II BASIC after the MEMORY SIZE? prompt. Others say R/S L2 BASIC after a MEM SIZE? prompt. This indicates that there were two versions of Level II BASIC. Are the differences cosmetic like these or do they go lower?
 
RADIO SHACK LEVEL II BASIC after the MEMORY SIZE? prompt is probably Ver 1.1 ROM and
R/S L2 BASIC after a MEM SIZE? prompt is probably Ver 1.3 ROM. My Model 1 had Ver 1.1 ROMS
and had problems with Keybounce and lots of problems when using cassette to load programs.
I modified my Model 1 to have Ver 1.3 ROMS. It pretty simple to do, but requires one Resistor added
to the EPROMS. I've got a Document with exact instructions.

The MC14001 (NOR GATE) and MC14040 (12 bit counter) Mod is new to me. I haven't run across that
MOD yet.


Ian posted this years ago.

http://www.vcfed.org/forum/showthre...l-1-in-2016/page2&highlight=Model+1+Lowercase



I simplified it this way which has served me well since the 80s.

If your character generator (Z29) last 4 numbers are 3001 or 6670, then you do the mod Lowercase with Upper from Custom TRS-80

If your character generator last for numbers are 6673 or 6674, then you do the Radio Shack lowercase mod, from the same book.

All other LC mods are complicated or unnecessary in this day and age, and in fact when I buy systems with other mods I undo
them and set them up as listed, above.

For the record:
3001 : early systems, lowercase but no descenders, and a flying 'a' due to Motorola manufacturing fault (so they sold these
chips cheap to RS who figured they didn't need LC anyway)
6670 : mid-production systems, same as above but with no flying 'a'
6673 : late systems, custom part made especially for the M1 for Radio Shack and included with most late 79 and 1980 machines.
By this time RS was selling the LC modification "kit" for $99 but if the computer already had the 6673 then the 1-min job
was to cut the trace, install the stacked 2102s into the already fitted socket, and solder two wires. It probably took
longer for the soldering iron to warm up than to make the mod.
Click to expand...

Larry
 
I know the previous owner had his Model 1 serviced, there is a Radio Shack service sticker on one of the bottom screw holes. One of the seven video SRAM chips is socketed and uses a different part than the other SRAM chips. There is another sticker under that one, but I see no need to damage the stickers more than they already are (I had to cut a hole in them to get to the screw.)

The bodge board is nowhere near the SRAM, so whatever it does, it is probably not a lowercase mod. A timing fix perhaps?

Thanks to those links, I know my ROM version is a 1.1 or 1.2. Probably a 1.2 as there are chips on this board with date codes of 8001 and 8003, suggesting this board was put together in early 1980.
 
The small board is an XRX-III, a weird fix that cripples the cassette hardware (breaking all the high-speed cassette utilities like B17) as a way to compensate for a poor choice of timing constants in the cassette-reading software on the early Level II ROMs.

See SAMS for a schematic. If you stick with the old ROMs, you probably want to leave this board in, for more reliable CLOADs. If you ever upgrade the ROMs, you may want to remove the board so that you can play around with high-speed cassette software.

Edit: to be more specific: I think the only ROM that has the fix that renders the XRX-III obsolete is the 8k ROM A/B labeled part 8044364 (earliest date code I've seen was 8005), in which the delay-loop constants at addresses 0249h and 0250h were changed from 41h and 76h to 60h and 85h. The MEMTEST utility (only included in double-density TRSDOS) identifies this ROM by the checksums B078 and DA45.
 
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