DEC VT20 boot device

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. Apparently the VT20 used the M792-YK as its bootstrap; the Field Guide is silent regarding the boot device and M792 documentation stops earlier in the series of variants. According to the VT72 print set, it used the MRV11-VC (M9942-YC; described in the Field Guide as a "bootstrap/diagnostic module") for its bootstrap but is also silent regarding the boot device. In interestingly, the Field Guide also describes a MRV11-AA (M7942-TB) as a "M7942 with VT52 emulator, VT71 bootstrap". For async. communications the VT20 used a DL11-B (M7800 (EIA)). the VT72 a DLV11-F (M8028). 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? paul