DEC VT20

DEC VT20
Manufacturer	DEC
Model	VT20
Communication
Interface	RS-232C

The DEC VT20 terminal is a variant of the DEC VT05 terminal with special facilities for typesetting.

The VT20/B is another variant with a different enclosure.

Two VT20/B terminals were connected to single PDP-11/05 (a variant of the 11/10) which was connected via RS232 to a host system, either a larger PDP-11 or a DECsystem-10, running Typeset-11 or Typeset-10. Newspapers using Typeset-10 were The Kansas City Star, the Chicago Tribune, and the London, Ontario, Free Press. The PDP-11/05 was booted by toggling in the bootstrap using the switches on the front of the machine, then downloading the abs loader and actual software through the RS232 interface from the host. The PDP-11/05 buffered text (news stories) downloaded from the host and allowed editing on the VT20. On Typeset-11 it was page oriented, a page would be downloaded, edited, then uploaded back to the host. On Typeset-10 the text was downloaded, but an associated memory system mirrored changes made on the text in the -11 to a copy on the -10. When the "save" button was pressed, the text was copied from associative memory into the actual text file on the DECsystem-10. This allowed for virtual scrolling through large files without having to save and load pages. The VT20s were eventually replaced by the VT72s, which featured a micro PDP-11 internally, with twice the memory of the old PDP-11/05s that controlled the VT20s. Interestingly enough, Digital no longer supported the associative memory version of the editing software, requiring page level editing. The Kansas City Star rewrote the PDP-11 software for the VT20s to run on the VT72, allowing for further virtual scrolling of large files, and making saving edits faster. The VT72 was replaced by the VT172, virtually the same terminal but in a VT100 type of form, with even more memory (32k).

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