View Full Version : SOL-20 & S-100 questions

Hugo Holden
September 7th, 2018, 03:28 AM
(Excluding the Helios drives) I see that the SOL-20 computer was able to connect to an external 5.25" hard drive. I assume to do this it would have had to have the "Micro-disk" S-100 controller card fitted ? And when it did, would it connect to something like an IBM compatible external 5.25" drive, or was it a special type of 5.25" floppy disk drive that was used ?

Or to put the question another way; if I had a SOL-20 computer, with no other S-100 cards in it than a memory card, what would be the hardware requirement I would need to acquire, to set it up with an external disk drive ?

September 7th, 2018, 05:26 AM
You'd need a disk controller. The North Star single-density (and perhaps also the double-density) were apparently popular choices -- they were very popular when introduced, so it makes sense they'd get used on the SOL-20 as well. They do indeed work with standard Shugart interface drives such as used on the IBM PC. And, nowadays, we have Mike Douglas's Virtual Sector Generator:


You can even see the back of his SOL-20 in the picture :) The North Star controller wants hard-sectored disks, which are getting very hard to find, but Mike's little board generates the correct sector pulses from soft-sector (e.g. IBM compatible) floppy diskettes. Note that we're talking the disk *media* here, not the drives.

Other disk controllers will work, too. The only limitation is that you're running with a 2 MHz 8080, so unassisted double-density is not going to work very well. You could go with an older single-density controller like the VersaFloppy 1, or you could go with an "intelligent" (e.g. has its own CPU) floppy controller like the Jade Double D.

Hugo Holden
September 7th, 2018, 11:49 AM
Thanks Glitch, That is great. I just bought a North Star disk controller card from ebay , picture attached (I can repair the hardware side of things if there are issues) and I bought one of Mike's boards.

Can you advise another thing : In the photo on Mike's website that shows some drives, what model are they ?

Also, I know that references to ebay are not encouraged, but, if you were buying an external 5.25 drive to use with Mike's board and you wanted a likely reliable and excellent result, could you point me to one or two examples of quality units ? Also are there 3.5" dives that might work ?

Dwight Elvey
September 7th, 2018, 12:10 PM
If you plan on using soft sectored disk with a sector pulse generator, the floppy drive should not be a belt drive. Most any of the direct drive have good speed control. The N* controller has tight margins on sector timing. Still you will find others have low level bios drivers for the N* / SOL combination.

Hugo Holden
September 7th, 2018, 12:37 PM
If you plan on using soft sectored disk with a sector pulse generator, the floppy drive should not be a belt drive.

When I got my 5155 I removed both the 5.25 drives and replaced them with a ST251 HDD and a dual 3.5/5.25 floppy drive with a Unique Floppy controller card. I put the two 5.25 drives into storage, I did not inspect them to see if they were belt drive or direct drive. I wonder if I could use those (if they were a direct drive type ?) if I built them into a case with a psu ? Have any projects like that been done on the forum before ?

September 7th, 2018, 07:24 PM
If you’re not picky about proper vintage, then get a couple of half height, direct drive, 48 tpi drives like the Teac 55BV series. These work great as DSDD drives with the North Star controller and the VSG. They’re also easy to power with an external supply like this: https://www.amazon.com/dp/B000MGG6SC/ref=psdc_10967101_t1_B00D2LS2AO (pick up a Molex Y-adapter to connect power to both drives).

If you want full height drives, I’d go with a couple of SA-400’s. They’re single sided, but handle double density without any problem. They’re pretty easy to restore and re-align. Even though they’re belt drive, I’ve never had an SA-400 that didn’t work well with the VSG. Numerous disk images for CP/M and North Star DOS on my website are single sided. You may need a separate power supply per drive if you go the external power supply route mentioned above.


Hugo Holden
September 7th, 2018, 10:47 PM

Thanks so much. I just looked at the wonderful resources for the SOL-20, even the .WAV files for the original cassettes!

A question about the ROMs in the SOL-20:

If there are faults on the computer I should be able to repair them and I have plenty of spare 74 series TTL IC's.

I always wonder though about the integrity of the numerous ROM's in the unit on the main board with the "personality module" and also some on the Disk controller card too. I see you have the .ASM files for some of the ROMS, and I wondered are the .BIN files available for the ROMS too ?

Or is it recommended to use the ASSM 8080 assembler to regenerate them ? If that were the case though, it would require an already working SOL-20 or similar 8080 machine to run the assembler. So if the machine has failed due to a faulty ROM it could be difficult without the .BIN files to program a replacement ROM ?

September 8th, 2018, 06:06 AM
The only ROMs in a SOL are typically the MOS PROM(s) on the personality board and the TTL PROM(s) on the floppy controller. Failure of these PROMs is not common. Standard EPROMs can be used on the personality module if needed. See http://deramp.com/downloads/processor_technology/sol-20/hardware/Sol-20%20Restoration.pdf which contains a section about this. The TTL PROMs are harder to find and program. I found a guy in Canada who has a good inventory of TTL PROMs and can program them if you have a bad TTL PROM on the floppy controller.

You can assemble the source files under CP/M using ASM and generate .hex or binary files as needed. For development, I run CP/M under SIMH on a PC, or run CP/M on another vintage computer.


Hugo Holden
September 8th, 2018, 11:51 AM

Thanks that is really helpful.

Hopefully I won't have any PROM issues, but I always try to think ahead to plan what to do if there is trouble.

I the meantime I found where I had stored the 5.25" drives. The two drives I removed from the IBM 5155 are belt drive types and the belts don't look great either, the rubber is degrading, but lo & behold I have a new old stock Panasonic 5.25" drive which is direct drive. (It even still has the cardboard protector where the disk goes). I must have got it as a spare for the 5155.

So I'm going to put this drive in a nice enclosure with a PSU. I want to put the VSG in the enclosure.

It looks from the VSG schematic that there is no reason that I cannot run the VSG from the supply for the drive and put it in the same enclosure, although I wondered is it better that the VSG gets powered before the drive (because it is waiting to detect the drive is on for the battery saver function), or would it not matter with the timing of power up ? What is the max voltage in this case you would recommend to the LT1121 regulator in the VSG, would 12V be ok? I could always feed it from a 9V linear regulator if not.

Hugo Holden
September 8th, 2018, 12:46 PM
.... I also notice that most of the newer direct drive 5.25" drives, their hardware specs are 96 tpi, I assume they would still be ok for this application, or is there a reason in this case why the 48 TPI drives would work out better ?

September 8th, 2018, 12:51 PM
Running from the drive’s 12v supply should be fine for the regulator and for circuit operation.


Hugo Holden
September 8th, 2018, 02:51 PM

I found two Panasonic direct drive drives, the NOS one is a JU-475-5 AKJ which I think is a 1.2 MB 96 tpi type and the other is a used & dusty (will need cleaning) is a JU-455-7AAB type which I think is a 320kB type, I guess that one would be suited, but as yet I cannot find the data for its jumper settings. I will clean it up in the meantime. I may be better off getting the Teac 55BV types suggested.

September 10th, 2018, 06:00 AM
I've had a lot of Teac drives in the shop with very few problems, but the Panasonics will probably be fine.

September 12th, 2018, 03:04 AM
Just a thought, what ever happened to the idea of a jig to make hard sector disks out of soft sector to maintain compatibility with the original setup as it was back in 76/77 instead of trying to find a drive that could be compatible with soft sector disks running a hard sector emulator?

September 12th, 2018, 04:06 AM
Just a thought, what ever happened to the idea of a jig to make hard sector disks out of soft sector to maintain compatibility with the original setup as it was back in 76/77 instead of trying to find a drive that could be compatible with soft sector disks running a hard sector emulator?

It's a pain in the butt, and you almost always end up with a dimple in the disk where the punch goes through, which rubs the jacket as it spins. Mike's solution is better I think. The vast majority of newer drives are compatible, I've even used the VSG with my old belt-driven Tandon TM-100s, it's just a less-recommended setup.

Hugo Holden
September 12th, 2018, 01:16 PM
Just a thought, what ever happened to the idea of a jig to make hard sector disks out of soft sector to maintain compatibility with the original setup as it was back in 76/77 instead of trying to find a drive that could be compatible with soft sector disks running a hard sector emulator?

That is an interesting idea, how many extra holes would be needed ? I'm pretty sure I could punch the disc material well with a clean flat edge (as I can admit I've used old 5.25" disks to make thin washers for various engineering applications) but what I think the hard part would be, would be spacing the holes accurately around the circumference of a circle and having them exactly the same size as the existing hole. It would really require a jig with the punch fitted and a precise rotation system. if they were not right, wouldn't that produce a timing-phase error ?

But now, an idea; most plastics companies cut plastic effortlessly laser and CAD. In that case, the alignment and potential hole edge problem would be solved together. If anybody had an old hard sector disk, if it could be posted to me, I could take it to my local plastics/laser company and investigate doing it this way.

Dwight Elvey
September 12th, 2018, 02:04 PM
I have a drawing someplace, for my punch, showing the diameter of the position of the holes. They are .100 inch diameter.
It is also hard to make sure the disk are correct. If it doesn't have a donut in the center, it is a 1.2M disk and can't be used.
If it does have a donut, it is likely a 360K disk but not always. It may still be a 1.2K and will cause errors.
I still use my punch. If it causes problems with burrs, I smooth them down with the back of my finger nail. It is not
an issue.

September 12th, 2018, 05:24 PM
Years back companies made kits that enabled you to put your own reinforcing rings on disks that didn't come with them right out of the box. IIRC, mine was purchased from Inmac but there were others.

As a result I wouldn't assume anything just because a disk had a reinforcing ring.