> On Aug 1, 2026, at 4:19 PM, Bill Degnan <[email protected]> wrote:
>
> Dave Gesswein constructed electronics to read the tape data into his laptop
> directly from a TU56. We backed up a pdp8, 11, and 10 tape all with the same
> process. Dave used a program that I think Vince Briel created, but I don't
> know who for sure, to view the tracks individually after import. This
> program would show which tracks were all 00's and which were data. The
> number of 00 tracks demonstrates whether the tape is 12, 16, 18 bit but I
> don't recall exactly how this is done other than to count how many columns
> were all 00's. We also viewed wave forms of the data as it was being
> inputted.
Nice. Is there a description of this somewhere?
Your comment about 00 tracks does not appear correct. In all DECtape formats,
there are 10 physical tracks, which are two copies of 5 tracks: a timing track,
a mark track, and 3 data tracks. This is true for all formats, not just PDP11
(which is the same as PDP-10) but PDP-8 also. The PDP-8 format has 4 3-bit
word per PDP-8 word; the PDP-10/11 format has 6 3-bit words per 18 bit word,
where in the PDP-11 case the top 2 are ignored on read and writtten as 0.
The way you recognize a PDP-8 format tape is by its short blocs. The PDP-11
format has 256 18-bit words per block, 578 of those per tape. The PDP-8 format
has 129 12-bit words per block, and I think 1102 blocks per tape but I'm not
sure about that. The mark track gives the block envelope which is how you can
tell them apart.
The reason you can read odd format tapes on a PDP-11 is that the mark track
definition is common across all of them. And this is why you can't read
LINCtape that way, its mark track codes are different and the block envelope is
different as well. For example, you can read DECtape in either direction, no
matter what direction the block was written in. But LINCtape can be
written/read in either direction but a block can only be read in the same
direction it was written.
paul
