General TC11 DECtape diagnostic/formatter questions

Paul Koning paulkoning at
Tue Nov 8 14:54:05 CST 2016

> On Nov 8, 2016, at 3:37 PM, Josh Dersch <derschjo at> wrote:
> Hi all --
> ...
> 2) I'm looking for means to format DECtapes on the TC11.  I have a few
> marginal tapes and I'd like to see if reformatting them brings them back to
> life.  The maintenance manual only indicates "a special program supplied
> with the TC11 system," and I haven't managed to find it. I *have* found
> this:
> which I've assembled and run on RT-11 and it goes through the motions of
> writing out the timing and mark tracks, but when it goes through the second
> pass to write out the block numbers it fails immediately, with either
> status 001207 (indicating a "Data Missed" error) or 020033 (Mark Track
> Error).  I haven't yet hooked up a scope to see if the T&M tracks are
> *actually* being written, but given my experience with the diagnostics in
> (1) above, I'm not averse to thinking there may be more than meets the eye
> with this issue.
> So in a nutshell:  Anyone used a TC11 on a later PDP-11 (like the 11/44)?
> Anyone have any thoughts on the diagnostics and formatter issues?

In college I used them on 11/20 and 11/45 processors with no troubles.  And I think at DEC we had them on an 11/70.  In any case, there's no reason to expect trouble based on the CPU type.

The standard DEC supplied formatting program was originally supplied as a paper tape diagnostic.  Way back in 1974 I rewrote it slightly so it would do the whole job in one pass, without asking you to reset switches, and it would also write proper empty directories (DOS style).  But the standard program should work fine, and the one you mentioned appears similar.

It's sufficient to have both the WRTM and WALL enable switches set for the whole operation.  The official approach is to set WRTM (only) during the first pass, WALL (only) during the second, and no switches after that.  But having unnecessary switches set to enable is harmless.

If you had a switch set wrong you'd get an invalid operation error.  You're seeing some different code, which suggests either the mark or timing tracks weren't written properly, or that there's some issue with the read circuitry.  Yes, I would say it's time to hook up the scope and start tracing some signals.

If you have a tape believed to be good that you're willing to erase, you might try reformatting that one.  That would help rule out issues caused by bad media.  While it is very rare for DECtape media to fail to the point that formatting doesn't work, it *is* possible.


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