Which Tarbell Controller? There were 4 revisions of the single density board, A thru D, and way too many revisions of the double density board (A thru H, but A thru D did not work reliably; the revision "E" board was really the first version of the double density board that was reliable). The single density Tarbell board was a mess, it wasn't much more than a disk controller "prototype" board, you basically had to jumper-wire the entire board yourself by hand based on the type of drives that you had, it is just awful (typically, you ended up installing something like 50 jumper wires). HOWEVER, if you got the board jumpered right (and, of course, if the parts were all good and if it was assembled right), it actually worked really well. The double density boards ran way to hot, the voltage regulator was way over-stressed, and I always put a low-value 2-watt resistor across the voltage regulator on the back of the board (something like 10 ohms) to relieve a bit of the stress on the regulator. The single density board was an "early" item, before the drive interfaces became relatively standard. Because of the jumper options, it could be modified to take ANY drive. The double density board basically requires either a Shugart type drive (or one very compatible with it), or a Persci drive. There were only a relatively few jumper options. Manuals for both boards are on Howard's site.