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From: jnc@mercury.lcs.mit.edu (Noel Chiappa)
Subject: Re: [TUHS] core
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    > From: Johnny Billquist

    > incidentally have 18 data bits, but that is mostly ignored by all
    > systems. I believe the KS-10 made use of that, though. And maybe the
    > PDP-15.

The 18-bit data thing is a total kludge; they recycled the two bus parity
lines as data lines.

The first device that I know of that used it is the RK11-E:

  http://gunkies.org/wiki/RK11_disk_controller#RK11-E

which is the same cards as the RK11-D, with a jumper set for 18-bit operation,
and a different clock crystal. The other UNIBUS interface that could do this
was the RH11 MASSBUS controller. Both were originally done for the PDP-15;
they were used with the UC15 Unichannel.

The KS10:

  http://gunkies.org/wiki/KS10

wound up using the 18-bit RH11 hack, but that was many years later.

      Noel