On the Emulation of TU58s

There's no timing track in the TU58 scheme, but there are magnetic BOT and EOT markers that have to be written so that the drive can identify the tape ends. I assume there are also block headers much like on most floppy disks, but I haven't gotten that deep into the formatting yet. The tape includes the BOT and EOT sensing punched holes, and the cartridge includes the angled mirror behind the tape to allow the holes to be sensed with a right-angled optical path. But the TU58-XA drive mechanism does not include the optical sensor that would be needed to sense tape ends on an unformatted tape. I don't know if this was meant as a way to further cost-reduce the already mechanically simple tape drive mechanism, or if DEC did that deliberately to make sure that non-DEC DC100/DC150 cartridges could not be be formatted in the field, so that users would be stuck buying preformatted cartridges from DEC. If new cartridges with brand new, un-decayed belts could be manufactured, then it should be possible to hack up a TU58-XA mechanism for formatting them. I think there may be a little hole in the plastic casting of the drive where one of the optical sensors might be glued in place, if I recall correctly. Oh yeah, the metal vs. plastic base cartridges were also mentioned in this thread. I've only encountered the metal ones so far. Based on the manual pictures, I think the plastic ones use a shorter belt with a simpler path. -- Mark J. Blair, NF6X <nf6x at nf6x.net> http://www.nf6x.net/