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TRS-80 Model 1 ROM adapter supporting 4 banks

M

mrthreeplates

Experienced Member
Joined
Jul 22, 2018
Messages
114
Hi,

I recently acquired a TRS-80 model 1 and was inspired by Ian Mavric's FreHD ROM mod supporting switching between the stock level 2 ROM and a modified version supporting autoboot.

Before I got a chance to install it, I happened to acquire a second TRS-80 model 1 with some mysterious switches. It turns out that one of the switches selected between Level 1 and Level 2 ROMs! I thought this was pretty cool as I could play with the somewhat different BASIC supported by Level 1.

After studying Ian's mod, I wondered if I could extend this idea to support additional ROM images.

I was half way through designing an adapter when I stumbled across a ready made (and customizable) one that is nearly perfect.
See 23xx-adapter from Retro Innovations. All I needed was a larger 27C512, and a few odds and ends.

Here is the unmodified 23xx adapter:

PXL_20210124_183830489.MP.jpgPXL_20210124_184007886.MP.jpg

Here is the schematic:

PXL_20210124_183758344.MP.jpg

I only needed to make the following changes:

* Cut out R1, R2 and R5 (the pull ups for A11, A12 and A13)
* Cut out IC4 (unused and not needed)
* Bridge the solder jumpers at SJ2, SJ4 and SJ5 (connecting ~OE, A11, and A12)
* Connect wires to ~CS, A13, A14, A15 and GND
* I also added a bypass cap for good measure

I kept the pull-up resistors for A14 and A15.

Note, the soldering is kind of tricky. If you take your time and have a fine tipped soldering iron, it isn't too bad.

Here is the adapter all finished:

PXL_20210124_193412088.MP.jpgPXL_20210124_201345526.MP.jpgPXL_20210124_204446871.MP.jpg

And here is the adapter installed:

PXL_20210125_001747153.MP.jpg

BTW: my goal was to minimize the soldering on the motherboard. In this case I only had to solder the A13 wire, everything else is reversible by just pulling out the adapter.

I should probably point out that I have a rev G motherboard. This likely makes the installation a bit easier. In my case, I only needed to deal with the additional chip select for the second ROM image. This is accomplished by shorting the chip selects between the two sockets (see the blue wire running from the adapter to pin 20 of the second ROM). I could have also modified the switches at Z3, but I figured the jumper was more reversible. Connecting the chip select lines allows the EEPROM in the adapter to be used for the entire 12K of the ROM address space.

Note that other (older) motherboard versions might require different installation steps.

I also installed machine pin sockets so that the adapter would be held securely in place.

All that remained was for me to program the 4 ROM images. I used Ian Mavric's Autoboot ROM for the first two images (at 0x0000 and 0x4000) and used identical copies of the Level 1 ROM images for the last two (at 0x8000 and 0xc000). This allows me to select Level 1 or Level 2 ROMS by toggling the switch connected to A15. If Level 2 is selected, then the switch connected to A14 selects FreHD autoboot or not.

I haven't yet mounted the switches as I'm still thinking about the best location. I might change the ROM images in the future. Using an adapter means I can easily reprogram the EEPROM.

The one problem I ran into is that the 23xx adapter is slightly too tall and presses on the bottom of the TRS-80 case. I'm still able to close up the case, but am not happy about the pressure this puts on the motherboard and case. Ideally, the adapter should be redesigned to be lower profile to eliminate this problem.

Anyway, just thought I'd share for others who might be interested.

A huge thanks goes to Ian Mavric for providing the Auto boot rom and images (see downloads)!

Cheers,
 
Just curious if your Model 1 rev G Motherboard had Version 1.3 ROMS. I updated my Older Model 1
from Ver 1.1 ROMS to Ver 1.3 ROMS to get rid of repeating keystroke and trouble loading from Cassette.
Has worked for years, and was the best thing I did other than adding the Lowercase Character Kit.


Larry
 
Yes, my first trs-80 came with ver 1.3 I believe (since it had R/S in the boot message). I assume the Frehd auto boot version is also based on 1.3 as well.
 
Hi - Please may I use your post for an article in the March edition of TRS8BIT?
It's something I'd never even thought of. You could even have a Level 3 option or even a debug facility!!!
 
I was checking out YouTube when this post from Adrian's digital basement caught my eye: a diagnostic rom image for either a m3 or m1:

As I happen to have one free rom slot in my 4 bank adapter, this diagnostic rom is perfect! Although I don't currently have a ram issue (that I know of), this is going to be handy to be able to boot with a flick of a switch.
 
BTW: Here is how I ended up mounting the switches controlling the 4 banks of roms:

foo.gif
PXL_20220609_013103282.MP.jpg

Now that there is a diagnostic rom option, I'll have to re-do the labels.
 
mrthreeplates said:
Hi,

I recently acquired a TRS-80 model 1 and was inspired by Ian Mavric's FreHD ROM mod supporting switching between the stock level 2 ROM and a modified version supporting autoboot.

Before I got a chance to install it, I happened to acquire a second TRS-80 model 1 with some mysterious switches. It turns out that one of the switches selected between Level 1 and Level 2 ROMs! I thought this was pretty cool as I could play with the somewhat different BASIC supported by Level 1.

After studying Ian's mod, I wondered if I could extend this idea to support additional ROM images.

I was half way through designing an adapter when I stumbled across a ready made (and customizable) one that is nearly perfect.
See 23xx-adapter from Retro Innovations. All I needed was a larger 27C512, and a few odds and ends.

Here is the unmodified 23xx adapter:

View attachment 1068805View attachment 1068806

Here is the schematic:

View attachment 1068804

I only needed to make the following changes:

* Cut out R1, R2 and R5 (the pull ups for A11, A12 and A13)
* Cut out IC4 (unused and not needed)
* Bridge the solder jumpers at SJ2, SJ4 and SJ5 (connecting ~OE, A11, and A12)
* Connect wires to ~CS, A13, A14, A15 and GND
* I also added a bypass cap for good measure

I kept the pull-up resistors for A14 and A15.

Note, the soldering is kind of tricky. If you take your time and have a fine tipped soldering iron, it isn't too bad.

Here is the adapter all finished:

View attachment 1068807View attachment 1068808View attachment 1068809

And here is the adapter installed:

View attachment 1068810

BTW: my goal was to minimize the soldering on the motherboard. In this case I only had to solder the A13 wire, everything else is reversible by just pulling out the adapter.

I should probably point out that I have a rev G motherboard. This likely makes the installation a bit easier. In my case, I only needed to deal with the additional chip select for the second ROM image. This is accomplished by shorting the chip selects between the two sockets (see the blue wire running from the adapter to pin 20 of the second ROM). I could have also modified the switches at Z3, but I figured the jumper was more reversible. Connecting the chip select lines allows the EEPROM in the adapter to be used for the entire 12K of the ROM address space.

Note that other (older) motherboard versions might require different installation steps.

I also installed machine pin sockets so that the adapter would be held securely in place.

All that remained was for me to program the 4 ROM images. I used Ian Mavric's Autoboot ROM for the first two images (at 0x0000 and 0x4000) and used identical copies of the Level 1 ROM images for the last two (at 0x8000 and 0xc000). This allows me to select Level 1 or Level 2 ROMS by toggling the switch connected to A15. If Level 2 is selected, then the switch connected to A14 selects FreHD autoboot or not.

I haven't yet mounted the switches as I'm still thinking about the best location. I might change the ROM images in the future. Using an adapter means I can easily reprogram the EEPROM.

The one problem I ran into is that the 23xx adapter is slightly too tall and presses on the bottom of the TRS-80 case. I'm still able to close up the case, but am not happy about the pressure this puts on the motherboard and case. Ideally, the adapter should be redesigned to be lower profile to eliminate this problem.

Anyway, just thought I'd share for others who might be interested.

A huge thanks goes to Ian Mavric for providing the Auto boot rom and images (see downloads)!

Cheers,
Click to expand...
Does anyone have a copy of the Auto Boot ROM used here. Searched all over the internet and can't find a ROM Image anywhere. I have a TRS-80 Model 1 with Level II basic that was not booting (stopped at garbled screen). After fixing the power and replacing Cap 43 on the clock circuit it still would not boot so I burned an EPROM with the diagnostic ROM developed by Adrian Black and others and it then booted and ran the diagnostic. So I'm thinking I have a one or more bad ROMs on the level II board. I would like to replace replace the Level II board with an EPROM but can't find the ROM image.
 
Got the 12k version of the 3 level 2 roms in one file and burned it to a 27256 EPROM for which I built a converter. The Model 1 booted up to the Memory Size ! prompt, but after a few seconds the white text changes to rainbow colors and is no longer sharp. So, another mystery to solve.
 
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