IBM
Altair Floppy Disk Redux
Altair Floppy Disk Redux
#Altair #Floppy #Disk #Redux
“deramp5113”
Creation of a new and improved Altair floppy disk system using only parts available when the original Altair floppy disk system was designed.
Video about the MITS Altair 8″ floppy drive hardware:
Video about the Pertec/Altair 3202 dual drive 8″ floppy disk system:…
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What a beast!
Had about a dozen of the Pertec 401. The direct-drive stepper motor for the spindle was their dumbest idea. Had to replace bearings in more than a few spindle motors. Noisy dern things. Never could get the two-board controller to function. Blamed it on bad PMOS shift registers. The Tarbell controller was the only solution to make them work. Never could make them function with a double-density controller. The Shugart 800s made the transition to DD just fine. Used the snot out of the CPMUG CRC program. Great work!
Your own version of the IWM maybe?
The Shugart 800 series was made with two different style chassis. The original version, had extra mounting ears, and could be stacked only vertically. The latter version omitted the extra ears and could be stacked vertically or horizontally. I had been "given" four od these drives, and I turned them vertically, and stacked three drives vertically (side by side) in a cabinet that had originally been set up for two drives in a horizontal position stacked vertically. I used them with the "Big Board" Z80 single board computer (1771 chip controller). Originally, they were written in the standard 128 byte sector format, holding about 1/4mb per disk. However, the controller could be programmed to write 3 1500 byte sectors per track, to hold about 3/4 mb per disk. You'd have to mod the CP/M disk routines to do this, but it would be compatible with the CP/M file system.
Your disk controller is similar to what OSI did on their floppy controller.
PIP and having the B: to A: is crazy in CPM
Does the new board run Altair BASIC without mods?
I can confirm without a doubt that the official Altair disk drive is a real headache. After getting the drive at a premium price, buying the FDC+, restoring it to perfect working condition, testing it with soft-sector disks and failing, then trying hard-sector disks and still failing (likely because, even though they’re new, the disks don’t work properly), I’ve finally given up 🥲
very very cool thank you for this content
Awesome! … Great demo. Really good work!
Oh, I'd have been SO tempted to add a full-disk write-through cache to that card! Probably not kosher, though?
The great thing about having ur own computer collection is the facts you can play with it like this
In the museum if u are lucky enough to be able to power on the exibit, playing with it like this would be out of the question
As a Wyse terminal fanboy…. I get a little extra from this video. 🙂
Fabulous video – thank you so much!
Long time no see! Another awesome video!
Are Harddisks with Altair, Imsai, etc. possible ?
Thank you, sir. I enjoyed it.
Wow, the early 8-inch Pertec drives were fragile, never saw an early one. I worked on the newer ones, and the Shugart drives.
My first floppy drive system was from North Star, and it used a 5 ¼ inch drive the SA400 and others.
The floppy drive controller was single density and used 10 hard sector floppies (90K). It was fascinating that the board occupied one kilobyte of memory at E900 and was a read only device! The first ¼K was the bootstrap ROM and the second ¼K was a copy in the double density version. The third 1/4K contained the status and read data registers, and the last was the write data. When reading or writing floppy data, it would place Memory WAITs until ready. Remember that I said that this was a read only device? Yes, it wrote the write byte by using the last ¼K as a write data in the address.
There was also a double density version of this thing (1850K). Plus, there was also a double density version of the floppy drives.
The read only took a bit to wrap your head around and came in handy for other tricks of the trade. LOL
The biggest issue I ran into was that the ALS-8 Firmware Module used the last 8K of memory of the 65K map. Also, came in handy when I needed to put RAM in its place and use a bank switching scheme to share the ALS-8 Program Development System (from audio cassette) in RAM with it.
So Motorola's EXORdisk-I, the earlier one which did not use the MC6852 looks like it has Pertech drives. But what is the controller? The card in the EXORciser is just a parallel port connecting to some kind of controller in the EXORdisk. Take a look at the EXORciser documents on bitsavers, maybe this is hardware you recognize.
Cool, you made an EXORdisk-II for Altair! I'm wondering when MC6852 was available vs. FDC1771- maybe both in 1976. AN764, an application note showing how to make a floppy controller is dated 1976 anyway. 1771 is easier on the software I guess. Commodore 1541 (at least the original, before the PLA) and Apple-II also used no special chips, but not IBM compatible formats.
deramp5113, I can't get enough of your content, so I subscribed!
Nice work! I especially enjoyed the '70s-style board layout (other than the generous ground fill under the regulator).
This looks really interesting . Thanks for putting this together .