S-100 Bus Tarbell 1011A Single Density Floppy Disk Controller +Cable for Altair For Sale

S-100 Bus Tarbell 1011A Single Density Floppy Disk Controller +Cable for Altair

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S-100 Bus Tarbell 1011A Single Density Floppy Disk Controller +Cable for Altair:

I used my Altair for a long time with a Tarbell cassette interface and just loved it. So when Don Tarbell came out with a floppy disk controller, I just had to get one. Don's cassette interface was the best, but a floppy disk system with CP/M was like having wings. In researching this listing, I was saddened to learn that Don passed away May 19, 1998.
This is a model 1011D controller. I used it with a Shugart SA800 drive. The cable is about 35" long.
The board looks to be in good shape, with not a speck of dust. But under magnification, I do see that one corner is cracked on a 74LS175 at U40. I would guess it's still functional, but could be easily replaced if not.
All the chips are socketed. There's still flux residue on the back. It could've come that way from the factory or I might've built it as a kit if that was possible. Manuals and schematics are only a Google away.
The board is designed to work with a variety of floppy drives and is configured by soldering jumper wires. All these jumpers look like rework wires, but they're not. The silkscreen clearly labels each endpoint Enn with digits for nn that are explained in the manual. I can see at least three different color wires, so I think I might've used it with different drives over time. A couple of green jumper wires have one end still soldered and one end hanging in the breeze, I guess because those were the factory-installed wires and I no longer wanted them in place.
I was writing a lot of BIOS code at the time and recall writing code for this board. You can still find it on one of the CP/M User's Group disks since I had a hand in making and distributing those at the time. I believe it's BVIOS.ASM on Volume 38 of the SIMTEL archives.
I think my goal was to write very tight assembler code so that the BIOS took as little space as possible, leaving as much precious memory as possible for other uses since the BIOS was always resident.
Many floppy controllers of the day could not quickly read sequential sectors around the circumference of the disk. For example, by the time you finished reading sector five, the head would already be part way into sector six. The controller would "blow a rev" if you tried to read sequential sectors, meaning that you'd have to wait most of one more revolution for the desired sector to come around again. With a spindle speed of 360 RPM, that was 167 ms!
I don't recall if this particular controller could read sequential sectors or not, but I can see from just the index of files on Volume 38 of the CP/M User's Group disks that I left tools there for finding the optimal sector interleave so you could read the whole track in as few revolutions as possible, ideally one. Volume 38 also has my FAST.COM and SPEED.COM utilities for speeding up floppy I/O on CP/M. Improved versions are on Volume 50.
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