1 Aug
2026
1 Aug
'26
12:32 p.m.
On Aug 1, 2026, at 4:19 PM, Bill Degnan
<billdegnan(a)gmail.com> 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