DEC VT20 boot device

Paul Koning paulkoning at
Sat Aug 10 14:39:33 CDT 2019

> On Aug 10, 2019, at 2:56 PM, Paul Birkel via cctech <cctech at> wrote:
> The DEC VT20 terminal apparently included a PDP-11/05 with a direct mapped
> character display and was intended for text editing and typesetting.  It
> seems to have been followed by the VT21, and then VT71/VT72, all three based
> on an LSI-11 (KD11-F).  There's a real lack of documentation about these
> online, although the VT72 does have a print set.
> ...
> Looking in a DEC "Options and Modules" listing I see VT20 bundles including
> Typeset-11 and DECset-11, and it appears that the VT20 could be configured
> with two displays & serial lines in a single 11/05.
> So . is the boot device in these systems the remote host via the serial
> line?  What protocol would that have been?  Something native to Typeset-11
> and DECset-11?

I've never heard of DECset-11, but I worked on Typeset-11 1978-1980.  The VT71 was the standard high end terminal for that system.  I saw a VT20 sitting in a corner of our lab, but it was collecting dust and I never saw it operate, or connected to anything.

As far as I know, no trace of Typeset-11 or TMS-11 (same software pretty much, ported to a stripped-down IAS instead of RSX-11/D) have been preserved.

It seems plausible that the download protocol would be DDCMP MOP mode, since that was a standard protocol supported by DEC for this purpose.  But it could have been something custom as well -- it's not something I was ever exposed to.

Yes, as far as I know the 11/05 controller for the VT20 could drive two independent displays.  The VT71 had a single display, the control processor was built into the terminal case.

I'm fairly sure the VT71 software was derived from that of the VT20, but just how close they were I do not know.


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