I wonder if it would be worth putting together something like the Kryoflux <http://www.kryoflux.com/?page=kf_features> for tape drives (the Kryoflux a device that connects directly to the floppy drive instead of through a floppy controller, and records the timings of the magnetic transitions at a low level, so that they can be used to extract data, or duplicate a floppy, in almost any format).
// Christian On 7 June 2015 at 03:47, Bob Supnik <[email protected]> wrote: > Yeah, PDP-7 (and earlier) DECtapes are different. They use different > checksum algorithms, for one thing, and some early PDP-4 and -7 DECtape > controllers used four-word headers instead of five. The checksum is > probably key; only five "four word header" DECtape controllers escaped the > factory. > > /* Checksum routine */ > > int32 dt_csum (UNIT *uptr, int32 blk) > { > int32 *fbuf = (int32 *) uptr->filebuf; > int32 ba = blk * DTU_BSIZE (uptr); > int32 i, csum, wrd; > > #if defined (TC02) /* TC02/TC15 */ > csum = 077; /* init csum */ > for (i = 0; i < DTU_BSIZE (uptr); i++) { /* loop thru buf */ > wrd = fbuf[ba + i] ^ DMASK; /* get ~word */ > csum = csum ^ (wrd >> 12) ^ (wrd >> 6) ^ wrd; > } > return (csum & 077); > #else /* Type 550 */ > csum = 0777777; > for (i = 0; i < DTU_BSIZE (uptr); i++) { /* loop thru buf */ > wrd = fbuf[ba + i]; /* get word */ > csum = csum + wrd; /* 1's comp add */ > if (csum > DMASK) > csum = (csum + 1) & DMASK; > } > return (csum ^ DMASK); /* 1's comp res */ > #endif > } > > This is documented in the 18b PDP DECtape controller, based on sources I > have for PDP-7 DECtape code from the late 60s; but for some reason, I never > published a paper on it. > > This is one reason why recovering DECsys required reading the DECtape on > the last working PDP-7 in the world. > > /Bob > _______________________________________________ > Simh mailing list > [email protected] > http://mailman.trailing-edge.com/mailman/listinfo/simh
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