Talk:DECtape

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[edit] Patent invalidated?

I'm skeptical of the claim that DEC's US patent 3,387,293 on DECtape was invalidated. The patent covers aspects of DECtape that are nontrivially distinct from LINCtape. For instance, LINCtape was designed for bidirectional search, but DECtape was designed for bidirectional data transfer as well. This required a substantial change to the way the mark track was encoded.

Google doesn't seem to have any other references to this patent being invalidated. Does anyone have a citation to the court case, or at least know who the plaintiff was? If not, I think it should be removed from this page and the LINC page. --Brouhaha 00:44, 19 Oct 2004 (UTC)

No citation for the patent being invalidated has been forthcoming in almost two years, so I'm removing that claim. I also plan to reword the paragraph comparing LINCtape and DECtape, as it is far too POV at the moment. There were legitimate reasons for some of the major differences between the two, and the patent covers those differences. --Brouhaha 23:33, 25 August 2006 (UTC)

[edit] DECtape on PDP-1 and PDP-10

I'm putting back the reference to DECtapes on PDP-10s. The KA10 system that I grew up on definitely did have DECtapes. Also, the PIP10 utility on OS/8 was specifically designed to allow PDP-8s to read and write PDP-10 DECtapes.

Atlant 18:22, 8 Dec 2004 (UTC)

Well, but not all PDP-10 models came with DECtapes. Alas, my decsystem10 Reference Manual doesn't explicitly say if all KA-10/KI-10 configurations included DECtapes as a standard, so I don't know about that. (According to the [configuration file], only two of the three ITS KA's at MIT had DECtapes, for what that's worth.) The KL10-B models (TOPS-20) definitely did not include DECtapes. (KL-10A's had them, but they were attached to the console PDP-11 - not sure if any OS's for the KL10-A allowed access to them from the -10.)
The other thing is that according to the same book, PDP-1's, -4's and -7's had them too. In other words, they were probably an available I/O option for pretty much all PDP's, so to be technically accurate we'd have to list them all, which would look silly.
In leaving only the 6, 8, and 11 in the list I had tried to list the older, smaller machines for which they would have been a primary mass-storage device - with their small (~300KB) size, once larger disks arrived, they wouldn't even have been useful for backup.
I would be OK with an edited list which didn't list all the machines that supported them, but just the ones for which they were a primary mass-storage device. Otherwise let's just link to PDP and be done with it... Noel (talk) 15:17, 9 Dec 2004 (UTC)

I've never seen a DECtape on a PDP-1, but if you say they existed, I believe you; this link certainly supports your contention:

http://groups-beta.google.com/group/alt.folklore.computers/browse_thread/thread/ebf63b298ba975b6/1d820bc0c5328407?q=PDP-1+dectape&_done=%2Fgroups%3Fq%3DPDP-1+dectape%26start%3D10%26hl%3Den%26lr%3D%26&_doneTitle=Back+to+Search&&d#1d820bc0c5328407

as does another newsgroup posting from Bob Supnik, who'd certainly know.

On the other hand, *EVERY* photo of a KA10 and every KA10 that I've seen first-hand had DECtapes, right up above and to the right the system console (not necessarily TU56 DECtapes; sometimes the older, relay-controlled TU55 DECtapes). It's a shame I don't have a copy of the Dick Best Options/Modules list any more. :) The PDP-1 apparently had the ancient 555 'tapes.

In any case, I'd be in agreement with omitting the list of all supported platforms and just referring people out to a PDP page and, if people felt it necessary, the LINC page (although LINC is linked from the PDP page).

Atlant 17:19, 9 Dec 2004 (UTC)

The KI10 diagnostics were supplied on DECtape; I suspect that DEC would not offer service on a KI10 without DECtape, just as they later required that KL10 systems have an RP06 to be on service. Early KL10 diagnostics were also on DECtape, but they were supplanted by the KLAD pack.

DECtape was not available until well after the introduction of the PDP-1, but was made available as an option for it. The PDP-1 being restored at the Computer History Museum includes an option to interface to an external Type 550 DECtape control.

--Brouhaha 22:06, 9 Dec 2004 (UTC)

A DECtape manual I have seen includes in the back a compabitily matrix. This lists every PDP family member and indicates what other PDP system's DECtape formats it can handle. So, for example, if you wanted to read a PDP-11 DECtape on a PDP-4, it would tell you whether that is possible (and how). Among other things, this implies that all PDP-n computers had DECtape capability. Paul Koning 20:38, 9 April 2007 (UTC)

[edit] Tape capacity?

The blocksize is 129 12-bit words in PDP-8 format, and 256 18-bit words otherwise. The TU55 book on Bitsavers [1] says the tape capacity (260 feet of tape) is 4096 blocks in PDP-8 format, which is 6.34 megabits. The number of blocks in the other format (PDP10/11/15 format) is 578, which translates to 2.66 megabits. This doesn't make sense. I'm positive about the 10/11/15 capacity, but the other comes from a DEC manual... did the length of the tape change? Paul Koning 20:56, 9 April 2007 (UTC)

  • More: the TU56 maintenance manual [2] says that the unformatted tape capacity is 2.7 megabits, density is 350 bpi, and length is 260 feet. So the PDP10/11/15 number seems right, which makes me wonder where that other number comes from. Paul Koning 21:02, 9 April 2007 (UTC)
    • Ok, the PDP-8 handbook [3] says 1474 blocks, which translates to 2.28 megabits. That fits the other numbers, and it matches the "184k words" mentioned at the top of the article. So the TU55 overview must be in error. Maybe the "4096 blocks" is meant to show the max addressable block number -- 12 bit block numbers -- which could be achieved with a custom format that uses a much smaller blocksize. Paul Koning 21:28, 9 April 2007 (UTC)

[edit] DECtape 2

I wonder if it's worth describing the lack of relationship between DECtape and "DECtape II". Some of us have long believed that the name "DECtape II" was a marketing ploy to freeload off the good reputation of the original. While DECtape II is block structured, it offers none of the key features of the real thing, in particular that of reliability... It also doesn't come with a reasonable interface, and it was only supported on PDP-11 and VAX. On some at least (RSTS for example) support was as minimal as possible and was dropped as soon as possible. Paul Koning 01:04, 10 April 2007 (UTC)

I've always assumed that it was mostly the block-addressability that earned it the name, but I also think that the Powers That Be hoped it would become a ubiquitous diagnostic medium much as DECtape had been ubiquitous. That's why we in the hardware world built it into essentially all of the VAX and PDP systems that were released around that time (11/750, 11/730, 11/44, and 11/24), but its obvious performance limitations (especially when compared to the even-cheaper floppy disks, even RX50 floppy disks) doomed it. So by the time of MicroVAX, and MicroPDP, floppies were the universal medium.
Atlant 12:07, 10 April 2007 (UTC)