What was a Terminal Concentration Device in DEC's products?

Chris Zach cz at alembic.crystel.com
Fri Jan 28 23:28:30 CST 2022

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?


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