On Jan 29, 2022, at 12:28 AM, Chris Zach via cctalk
<cctalk at 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
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
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.