unreadable HP cartridges

Tony Duell ard at p850ug1.demon.co.uk
Thu Jun 23 13:25:46 CDT 2005


> 
> Has the on-tape format of 98xx HP85 or 264x carts been documented somewhere?
> Did HP use different encoding schemes in their different product lines?

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.

> It may not be too bad with the HP DC100 style tapes, since I beleive the
> heads are fixed, like the DEC TU58s.

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



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