> On Jan 29, 2022, at 12:28 AM, Chris Zach via cctalk <cctalk@classiccmp.org>
> wrote:
>
> Old question: I'm looking through some old reports from 1977 about a failed
> DEC project with the DMX11 multiplexer system and there is reference to the
> following key items:
>
> 1) The DMX was designed to handle block mode devices. Fine.
> 2) Character mode devices like the VT52's were supposed to use a "TCD"
> product from DEC.
>
> The reason the project imploded was because apparently DEC stopped supporting
> the TCD in RSX11/D in late 1976, so someone in CSS had the great idea of
> agreeing to extend the microcode in the DMX11 to handle both block AND
> character mode devices. This did.... not work well and it sank the project.
>
> What I'm wondering is what was the TCD for PDP11's back then? I don't see
> anything in my communications handbooks on this, and even the DMX11 doesn't
> really appear, instead there is the COMM/IO/P type boards which worked with a
> pile of DZ11's. From what I can glean from this documentation it looks like
> the DMX11 worked in a similar fashion as the requirement was the DMX11 system
> was a nine board solution (possibly 8 DZ11's and one controller board).
>
> More odd it looks like the TCD *was* still supported in RSX11/M and
> ultimately the decision was made to build the thing in M so it's weird they
> continued to whack away at the DMX solution instead of going with TCD's for
> async and proven DMX microcode for block devices.
>
> Any thoughts, or does this jog any memories?
Nothing comes to mind here; the name "DMX" does not ring any bells. It's a bit
before my time, admittedly.
DEC made some products that used block mode terminals: the moderately
successful Typeset-11 with the VT-61/t forms and page editing terminal, and the
VT-71 with embedded LSI-11 to do full file local editing. Both have some form
of block transfer to the host, but as far as I can remember they used ordinary
DH-11 terminal interfaces. DH-11 is unusual in that it has DMA in both
directions, which is unhelpful for interactive use but great for block
transfer. Typeset-11 also supported a specialized terminal made by Harris (the
2200), another local processor device, this one connected to the PDP-11 host
with a DL-11/E, using half duplex multidrop BISYNC with modem signal
handshakes. I kid you not... I have some scars debugging that protocol at 2 am
in downtown Philadelphia.
DEC also built yet another VT-51 variation, the VT-62, which was the terminal
for the TRAX system. That was, I think, some sort of RSX derivative (-M+
perhaps, but I'm not sure), which made it to field test but was canceled before
becoming an official product. Not sure why.
paul
