unreadable HP cartridges

I ma pretty sure the 9825 and 9845 formats were somewhat similar, and that the 9815 was very different. Although all 3 machines used much the same tape drive (and the same PCB containing the head switching circuit, read preamp, etc). The hardware of the HP85 tape drive is somewhat documented in the service manual (on the MoHPC CD-ROM set). The hardware of the 9815 and 9825 is on the HPCC scheamtics CD-ROM. The former has one ASIC for all the tape drive functions, so you can't tell too much about it. The latter are all standard chips, but a lot of the work is done in software, alas. Thinking about it, there must be differences between the software interface to the 9825 and 9845 controllers. The former does DMA, with one bit at a time going to the LSB of a memory word (it does no serial-parallel translation!). IIRC, it only uses the low 8 data lines on the I/O bus. The latter doesn't seem to do DMA, but all 16 data lines go to the controller hybrid. Correct. I can't remeber if it's 2 tracks or 4 (I think the latter), but there's a schematic, and the drive itself does very little processing of the signals. One of the pins on the drive connector is the output of the read preamp (the main amplifier is on the controller board). -tony