IMSAI 8080 - Reboot

[pcjunky]

I am on a quest to get an IMSAI 8080 I have had for many years to boot and run CP/M. I am reaching out to anyone who might be able to help.

Below is a link to photos of the system and some S-100 boards I have avaible to accomplish this. I also have some external 8" drives (Lobo drives for an apple II) and a dual Micropolis dual 5.25 inch drive that originaly was run one a SOL-20 I also have. (Dave Greelish told me is used to be Stan Veits personal system). I also have an Altair 8800 system with several S-100 cards I would be willing to use on the project.

Not shown in the photos is a great deal of test equipment that could be brought to bear on this. Logic analyzers, Oscilloscopes, TTL chip testers, etc. I also have a large collection of IC's salvaged and saved over the years.

http://explorer.cyberstreet.com/IMSAI8080/

 

[atod]

I have an IMSAI as well, however have not brought up CP/M on it. I was able to program an EEPROM monitor to validate most stuff worked, then found out my old 8" drive heads were bad. This stalled the project and now I'm in NYC where my apartment is too small for this stuff (for the time being). How much of the system have you validated? I suggest starting in validating the CPU and Memory board are ok. You can do this simply by toggling bits in and writing them to various memory locations. I believe you will need at least 40K of contiguous RAM for CP/M. If you CPU board has a socket for a monitor ROM or perhaps GPIO board does, suggest you create a monitor ROM to do all of this from a serial console. I have some assembly for this if you are interested.

 

[billdeg]

I am on a quest to get an IMSAI 8080 I have had for many years to boot and run CP/M. I am reaching out to anyone who might be able to help.

Below is a link to photos of the system and some S-100 boards I have avaible to accomplish this. I also have some external 8" drives (Lobo drives for an apple II) and a dual Micropolis dual 5.25 inch drive that originaly was run one a SOL-20 I also have. (Dave Greelish told me is used to be Stan Veits personal system). I also have an Altair 8800 system with several S-100 cards I would be willing to use on the project.

Not shown in the photos is a great deal of test equipment that could be brought to bear on this. Logic analyzers, Oscilloscopes, TTL chip testers, etc. I also have a large collection of IC's salvaged and saved over the years.

http://explorer.cyberstreet.com/IMSAI8080/

On this very board there have been many discussions about how to get a s-100 system to boot disks. It's not easy and will require a lot of time and patience, or money.

Before you can boot the system to any disk OS you need a working core system than can read and write to 24k or more of RAM using the front panel. It goes on from there. If the RAM is bad you will be eternally frustrated.

Even before that yoou should know the power supply voltages are ok, check the s100 bus lines for proper voltages.

Bottom line ... You *can* do this, but it will take a patient committment. Front panel IMSAI and Altair computers are not turnkey systems, you have to match up the RAM, I/O card, processor, disk drive card, and use a CP/M gen that is compatible with all of these simultaneously.

As far as getting help from this board, you'll get the most effective assistance if you provide specific focused details about a well-defined problem. Name your photos so that people don't have to look through them all, make it easy for people to help you.

 

[MicrocomputerSolutions]

I have an IMSAI as well, however have not brought up CP/M on it. I was able to program an EEPROM monitor to validate most stuff worked, then found out my old 8" drive heads were bad. This stalled the project and now I'm in NYC where my apartment is too small for this stuff (for the time being). How much of the system have you validated? I suggest starting in validating the CPU and Memory board are ok. You can do this simply by toggling bits in and writing them to various memory locations. I believe you will need at least 40K of contiguous RAM for CP/M. If you CPU board has a socket for a monitor ROM or perhaps GPIO board does, suggest you create a monitor ROM to do all of this from a serial console. I have some assembly for this if you are interested.

Most surviving 8" floppy drives need a inspection and alignment before they can be put into service because of their age. Drives need to have their alignment checked and adjusted, just as cars need a periodic tuneup, and wheel alignment. You can't just go out, buy a pair of drives (even drives that are supposed to be unused, are still 25-30 thirty years).

You don't want to risk unreplacable Original Media (boot disks or applications program disks) in floppy drives of unknow origin, and working condition, that may render the disks worthless.

Some brands of drives have proven to be very reliable, and some brands of drives have proven to be unreliable, or feature fatal design and/or manufacturing/materials defects that make them less valuable/less easy to salvage and restore.

I have been providing service and repair on 8" and 5.25" floppy and mini-floppy disk drives since the mid 1970s, if you are ready to have yours evaluated, and repaired.

 

[monahan_z]

Actually FWIW I have not yet found most 5" or 8" drive that needed realignment. This is often quoted, but I must have worked with over a dozen drives in the past few years. Never once did they need realignment! Most were from eBay. Perhaps I was lucky. I’m sure if you drop one on the floor etc. it may be the case but in general they work fine if they can step properly. Also this is normally only a problem with already reading already formatted disks. It you format your own you should be OK (of course on a miss-aligned head nobody else could read it!). The only drives I would not vouch for are the old Persi drives which even when new had problems with their voice coil positioning.
BTW, I have had no difficulties also reading 30+ year old 8” disks that ware stored in a cool dry place. One bad out of about 100!

 

[MikeS]

Actually FWIW I have not yet found most 5" or 8" drive that needed realignment. This is often quoted, but I must have worked with over a dozen drives in the past few years. Never once did they need realignment!

Ditto that! Out of over a hundred or so 8" and 5.25" drives maybe two or three needed an alignment touch-up in order to be compatible with 'standard' commercial diskettes; aside from actual mechanical or electronic failures the only thing they needed, if anything at all, was cleaning the heads and cleaning and lubing the guide rails.

Same with my 8" diskettes; maybe 2 bad out of 100, and maybe they were improperly written in the first place.

But having said all that, it's certainly good advice to at least check the heads and test with a scrap diskette before you insert a rare or irreplaceable diskette; a damaged or badly contaminated head can instantly destroy every disk you put into it (and vice versa).

Quite an impressive and eclectic collection of cards, BTW!

 

[MicrocomputerSolutions]

Actually FWIW I have not yet found most 5" or 8" drive that needed realignment. This is often quoted, but I must have worked with over a dozen drives in the past few years. Never once did they need realignment! Most were from eBay. Perhaps I was lucky. I’m sure if you drop one on the floor etc. it may be the case but in general they work fine if they can step properly. Also this is normally only a problem with already reading already formatted disks. It you format your own you should be OK (of course on a miss-aligned head nobody else could read it!). The only drives I would not vouch for are the old Persi drives which even when new had problems with their voice coil positioning.
BTW, I have had no difficulties also reading 30+ year old 8” disks that ware stored in a cool dry place. One bad out of about 100!

If you have a running system, and you're just reading disks, alignment as not nearly as critical as if you have a non-running system, and you want to try sticking a Boot Disk in to see what will happen. Often times when people are trying to get a newly assembled system running, they stick their only (and the Original Master OS Disk) into the drive, because they don't have a running system to make copies to experiment with.

It doesn't make any difference why the system didn't boot up (system or drive/s configured wrong, defective disk controller board, or drive/s, or bad cables). It only takes the act of inserting the disk to cause damage that the User may not be able to repair.

Which is why I supply extra copies with every OS or applications disk I sell, and I recommend that Users don't write on any disk that they receive programs or data on. Instead, format disks and compy the OS or applications onto the newly formatted disk, unless you have paid to have your floppy drives professionally serviced, or have the tools to do your own professional grade alignment.

Lastly, there is more to servicing a drive, and simply checking the head alignment. The heads need to be cleaned and checked for damage or wear, and the head load tension needs to be checked. If it's a single sided drive, the head load pad probably needs to be replaced. The index pulse period and timing needs to be checked and adjusted, head settling time should be checked, and the track zero and inside track sensors need to be checked, and adjusted as necessary.

Drives always seem to be working, all the way up to the day when suddenly they don't seem to be able to read disk make before. And by then it's too late, because you've scrambled/written on your disks with an out of alignment floppy drive. If you have your drive/s serviced at that point, you can go on reading and writing to newly formatted disks, but the ones you wrote on previous to your drive/s failing may not be recoverable.

And I have 5.25" and 8" disks that I wrote on back in the 1970s, that are still in perfect usable condition. Proper storage is the key to the health of floppy disk media. And being able interchange between old and newly written disks is a matter of keeping your drives in proper alignment.