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mrthreeplates
July 14th, 2019, 07:43 PM
Hi

Is there a way to boot from the external floppy interface on a model iii?

I have a hxc floppy emulator and would like to boot from it without having to swap cables. Preferably, I'd like to be able to switch between booting from the internal and external interfaces.

Thanks

lafos
July 15th, 2019, 01:33 PM
I know of no way to boot directly from the external floppy. If you are using LDOS 5.3.1, you can boot from the internal floppy and then assign another drive to be the system drive. This is typically used to boot to a hard disk, but it can be done on a floppy, too. But both the internal Drive 0 and the external drive have to be running the same version of LDOS.

Petrofsky
July 15th, 2019, 05:03 PM
Is there a way to boot from the external floppy interface on a model iii?

Yes, sort of.


... without having to swap cables.
Oops. That was the way.


Preferably, I'd like to be able to switch between booting from the internal and external interfaces.


All the ROM knows how to do is load and execute track 0, sector 1 of drive 0.

You could burn your own ROM that scans the keyboard and then loads and executes track 0, sector 1 of drive 0, 1, 2, or 3 according to which of the '0', '1', '2', or '3' keys is depressed (defaulting to 0).

But what happens after the sector from drive 2 starts executing? It will likely try to load further data from the disk it came from, but it will do so by executing instructions to load data from drive 0.

The only way you can really be able to switch between having two different drives as the boot drive is to "swap cables", or install a physical switch that does the swapping.

mrthreeplates
July 15th, 2019, 07:49 PM
Thanks, yes that is what I was thinking as well.

My current thought is that I'll pull u6 from the floppy controller and replace it with a custom board. I'll have a switch that will flip the drive select lines making the change transparent.

Now I just have to make the mini board, and I'm a sw guy!

Patrick Bureau
July 17th, 2019, 05:36 PM
someone create a way to switch between drive 0 and drive 1, maybe you can adjust it to use drive/Gotek ? forget author name its not me, but its called Switch Select Floppy drives in a TRS-80 model 3 or Model 4.pdf
here is the document on a download link from my cloud storage page https://myaccount.dropsend.com/share/5ef0338929593587f9a99763eac4d57d/download?file_ids[]=15734433

lafos
July 17th, 2019, 06:11 PM
someone create a way to switch between drive 0 and drive 1, maybe you can adjust it to use drive/Gotek ? forget author name its not me, but its called Switch Select Floppy drives in a TRS-80 model 3 or Model 4.pdf
here is the document on a download link from my cloud storage page https://myaccount.dropsend.com/share/5ef0338929593587f9a99763eac4d57d/download?file_ids[]=15734433

This would work if the OP was willing to replace one of the internal drives with the HXC emulator. The internal and external drives are wired differently on the controller board. The first external drive is selected as Drive 0, and not Drive 2.

mrthreeplates
July 17th, 2019, 07:08 PM
Thanks for the link to that document! That is an interesting approach that I hadn't thought of, much appreciated.

Trash-Eighty
July 18th, 2019, 03:10 AM
I found two references related to this topic:

Hacking the Model III floppy interface to swap the internal and external drives (http://www.vcfed.org/forum/showthread.php?40887-Hacking-the-Model-III-floppy-interface-to-swap-the-internal-and-external-drives)

modification to the floppy disk controller to switch the drive select lines on the internal disk connector between drives 0 and 2 (http://www.quicktrip.co.nz/jaqblog/home/75-trs80m42/)

gonk23
July 18th, 2019, 10:37 AM
Hacking the Model III floppy interface to swap the internal and external drives (http://www.vcfed.org/forum/showthread.php?40887-Hacking-the-Model-III-floppy-interface-to-swap-the-internal-and-external-drives)

I never did get around to implementing my proposed solution. I didn't want to modify my FDC.


My current thought is that I'll pull u6 from the floppy controller and replace it with a custom board. I'll have a switch that will flip the drive select lines making the change transparent.

It sounds like you're thinking of the same thing as me.

Another idea I was thinking about was to build two long custom flat cables. One would connect to the top of the FDC (which is for drives 0 & 1) and run out to the bottom of the Model III. The second cable would connect the two internal floppy drives and also run out to the bottom of the Model III. You'd connect the two cables together (externally) if you wanted to use the internal drives as normal. Or you could connect the first cable (FDC) to the HxC if you wanted to use that instead. You could also potentially connect the second cable (internal drives) to the external connector so they could be used as drives 2 & 3. You'd have to work out suitable connectors at the external ends of the cables. It would be messy but it would avoid needing to modify the FDC or keep opening the case to swap the cables.

mrthreeplates
July 20th, 2019, 06:31 AM
Hi Gonk23, Trash-Eighty,

Thanks! I've ordered parts from digikey to prototype a breadboard so I don't have to do any rework on the floppy controller itself. I'll just have to socket u6 (at least on my version of my model 3's floppy controller). I am going to try to use a 74157 as a mux to make the switching simple. It is going to be a tight fit, so we'll see how it goes. Should be fun.

I'll report back my progress with pictures.

gonk23
July 22nd, 2019, 07:57 PM
I'll report back my progress with pictures.

Nice! I'm looking forward to seeing your solution.

mrthreeplates
August 10th, 2019, 10:38 PM
Success!

I used a small prototype board from chipquick as space was super tight. As I was worried about adding additional power to the system, I used low power cmos ttl compatible chips (HCT). Soldering was kind of tricky.

With the switch disabled, my drive assignment is unaffected as expected.

I flipped the switch, rebooted, and had the external drives swapped with the internal ones. Doing this allows me to boot directly off of my hxc emulator while still accessing the internal floppies.

Since the only mod to the floppy controller is adding a socket for u6, this mod is reversible.

My next step is to build a pcb. I have another trs80 and I'd like a second one of these boards. I think it would be cool to make the mod even smaller and I want an excuse to make a pcb anyway.

Comments or suggestions for improvements appreciated!

55204552195521655214552135521255211552105520955207 552065520555218

gonk23
August 13th, 2019, 01:16 PM
Nice! So what are the advantages of using the 74157 over just using a 4PDT switch?

mrthreeplates
August 13th, 2019, 07:20 PM
I am not sure there are any advantages as they are functionally the same (maybe less noise?). I just thought it a bit more elegant to use a simple logic switch vs. extending the drive select lines through a big physical switch. And I wanted to see if I could do it... just a software guy who is dangerous with a soldering iron!

gonk23
August 13th, 2019, 10:06 PM
I suspect the 74157 approach would be better if you needed to switch the drives while the computer is on (assuming that's a good idea in the first place). I suspect the physical switch can't be guaranteed to cleanly switch all the lines at the same time.