General TC11 DECtape diagnostic/formatter questions

Paul Koning paulkoning at
Wed Nov 9 15:05:33 CST 2016

> On Nov 9, 2016, at 3:32 PM, Josh Dersch <derschjo at> wrote:
> ...
> Now that I've gotten the full suite of diagnostics to run, the problem
> seems to be that the TC11 isn't reading properly in reverse -- Tests
> 15,16,21,22,26,27 and 34 of ZTCD fail, all others pass (modulo a marginal
> block on the tape causing a failure here and there).  This would probably
> explain why DTF fails immediately after writing T&M, since it works in
> reverse from that point...

Not necessarily.  I thought that reading direction simply changes the bit patterns seen by the electronics because the waveforms are reversed.  This is the famous "obverse complement".

In the Mark track, reversing the direction means that you see the bits in the opposite order and complemented.  As I recall, the encoding takes advantage of this: the start of block code is the obverse complement of the end of block code (so that reversing means the "end" code now reads like "start").

In the data, you have 3 bits parallel.  So there, reversing means that you get the 3 bits at a time reverse (think of octal digits reversed), complemented.  For 16-bit data, look at it as 18 bits with 2 bits unused.

In Read-All and Write-All, the controller doesn't do anything for direction, so if you write in one direction data meant to be read in the opposite direction, the software has to supply obverse-complement format data.

For regular Read and Write, the controller does handle direction change: the reverse commands do an obverse-complement on the supplied data words, so your data ends up word-wise reversed but the bits are in the expected spot and of the expected polarity.

The second pass (after the WRTM) in the formatting program is a reverse direction WALL, so my comment that the hardware doesn't do anything special for direction applies there.

A possibility is that the motor has a problem causing an excessive speed difference between forward and reverse.  But in any case, yes, scope and schematics seem needed here.


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