TPDD stands for "Tandy Portable Disk Drive." It is a portable (battery powered) diskette system for the Model T series.
There are two variants, the TPDD, and the TPDD-2.
The TPDD connects to the Model T via the serial (RS232) port.
Both the TPDD and TPDD-2 implement two different data access protocols. One mode is "Sector Access" and the other is "File Access."
However, the TPDD-2 and TPDD implement different protocols for the sector access mode.
Most Model-T applications use the file access mode. Brother knitting machines and some Model-T database applications use the TPDD sector access mode.
Aligning a TPDD drive
(From a Model 100 mailing list post by Steve Adolph)
Hey all, I just had a fun time realigning one of my TPDD. It was an interesting experience. I knew the TPDD was out of aligment because if created and used its own discs just fine, but could not read or write those formatted on other drives.
There is a single screw that clamps down on a position sensor that tells the drive when it has reached track 0. Making small adjustments to the position of that sensor is how you adjust alignment.
At first I advanced the sensor towards the spindle. I could not read/write known good discs.
Then I decided for fun to format a disk that had a good format on it from a good drive, thinking it would erase the old format and create a new one.
Later, I moved the sensor slightly away from the spindle. (truth is I fiddled with it back and forth, but ended up further away)
Voila - I could read and write to good floppies, and other drives could read files created by the tweaked drive.
What was cool was, the disc I had formatted when the drive was misaligned actually still had the original data on it!
Disassembly of the TPDD is not hard once you learn how to do it. To get at the screw, you have to remove the disc holding aparatus altogether.
Anyhow, that plus a new belt and it is ready to go again.