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TRS-80 Model 1 Expansion Interface for FreHD

J

jaquinn

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Joined
May 1, 2012
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91
Location
Auckland, New Zealand
It has taken a while but I finally got the PCB completed and tested for the Model 1 Expansion Interface for FreHD. This board upgrades a stock 16K Level II Model 1 to 48K and adds support for FreHD autoboot.

A write up along with design files are available at http://www.quicktrip.co.nz/jaqblog for anyone who wants to have a go building one and providing feedback. We are testing on a few Model 1's at the moment to make sure there are no compatibility or other undiscovered issues so any feedback from builders is appreciated.

The only known issue is that the V1 Schematic and PCB (included in the design files) does not pull up the FreHD /EXTIOSEL and /WAIT signals to VCC. This isn't a problem when the FreHD is connected because it drives these signals but when using the board without a FreHD these signals float. It doesn't seem to cause problems in my testing but adding two pull-up resistors is a good idea.
 
@ jaquinn

Good to see the " Quinnterface " up and running, Good work andrew,

Will you be making the Project available as a kit ??

I would like to make up one and do some testing.

Thanks Ray W___________
 
Hi Ray,

Yes it was a long time in the making (real life seems to get in the way a bit) but happy with the outcome.

Very likely Mav will make these available once he is happy that it meets the required quality standards.

I will do a board revision for the System/80 that you will also be interested in.

JAQ
 
Finally got around to building a version 1.2 board I had fabbed awhile back. Is the EEPROM image with the self test program Ian demonstrates in his video (SYSTEM /12288) available for download somewhere? I didn't see it in either the 1.2 or 1.3 design files.

Thanks, Chris
 
Please do not take this in a bad way but the interface as it stands is limited. And yes I know it was intentional to provide just memory and an FreHD interface. But it would have been cool if maybe a DD floppy interface could be attached like a kind of like a sidecar or something along that line. And ordered as an option?

Just an input. Totally up to you.
 
Last edited:
Chromedome45 said:
Please do not take this in a bad way but the interface as it stands is limited. And yes I know it was intentional to provide just memory and an FreHD interface. But it would have been cool if maybe a DD floppy interface could be attached like a kind of like a sidecar or something along that line. And ordered as an option?

Just an input. Totally up to you.
Click to expand...

Hi Frank

Absolutely right that the interface is limited. The design objective was to build just enough so the unloved Model 1's sitting in the back of closets could again have a slightly useful life... without the headaches of a full expansion interface, floppy drives and modification to support autoboot. I think we achieved that pretty well.

All the design files are available so if someone wants to attempt a real floppy controller then they are welcome. The Ciarcia Disk-80 provides a nice reference design.

By going down this path you do lose out on autoboot unless you had some way of switching between the real floppy controller and the PIC. It would be back to opening the Model 1 and replacing the ROM.

One of the things that makes this board so good for those that don't want to get inside the machine is the way the PIC pretends to be just enough WD1771 to provide a boot sector for the FreHD autoboot.
 
I don't want to "jump the gun" too much, but the prototype for something along these lines has been sitting on my desk for about 3 years now. It has:

- A real single/double density single/double sided floppy controller implemented via FPGA.
- Original-compatible RS-232 (DB9 connector) also in said FPGA.
- 32KB SRAM expandable to 128KB by bank selects (does anyone know of a "standard" for such bank switching I/O control on the Model I? I just made something up so it won't be compatible with any existing software).
- Heartbeat interrupt.
- RCA jack for cassette sound output, slightly filtered to sound less harsh.
- A unidirectional printer port (DB25 connector) compatible with the original.
- Expansion bus.

All of this has been finished and gathering dust for a long time. Recently I've finally started work on the grand finale:

- SD floppy emulation, planned to allow booting just about any DOS image, and intermixing real and virtual floppy drives (and hard drives) as desired.

Once this is done there'll be boards built and a Beta testing period, but it'll be a little while longer yet.

This was originally meant as a companion device for the MISE and got sidetracked for altogether too long. I showed the prototype to VCFMW last year and will drag it there again if I go this year. I also showed the USB device but most visitors to the exhibit just wanted to play some games.

Pete.
 
Hey Pete, Will you be using "card edge" connectors to connect to the Model 1? most likely would have to for compatibility reasons. Maybe use a better material than tin as in the original model 1 EI. Maybe copper?
 
The current plan is just to use exposed copper edge connectors for the expansion bus and floppies. There will also be a PCB site for a box header on each interface. My PCB fab can gold-plate the edge connectors, but the cost would be significant.
 
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