PCjs Machines

DEC VT100 Terminal with Advanced Video Option

This is our VT100 Terminal with DEC’s Advanced Video Option (AVO) installed. The AVO was a small board that plugged into the VT100’s terminal controller, and it added three things to the basic VT100:

• Enough screen memory to display 24 lines in 132-column mode (instead of only 14)
• Four more attributes for every character on the screen: bold, underline, blink, and an alternate character set
• Sockets for additional ROMs (program ROMs, and an alternate character generator ROM)

Trying It Out

The easiest way to see the AVO in action is to press the SET-UP key (or F9). With the AVO installed, the words “SET-UP A” blink in bold, the words “TO EXIT PRESS “SET-UP”” are underlined, and the tab ruler at the bottom of the screen alternates between normal and reverse video. Compare that to the SET-UP screen of a VT100 without the AVO.

While you’re in SET-UP, press 9 to switch to 132 columns and then press SET-UP again. You now have 24 lines of 132 columns.

You can also experiment with the attributes yourself. Press SET-UP, press 4 to switch from ON LINE to LOCAL, and press SET-UP again; now everything you type is sent directly to the screen. Then use the ESC button to type any of the following “Select Graphic Rendition” sequences:

Attributes can be combined in a single sequence (eg, ESC [ 1 ; 4 m for bold and underline), and they remain in effect for everything you type until you turn them off. The entire screen can be reversed as well, with ESC [ ? 5 h (and restored with ESC [ ? 5 l).

How It Works

As with the rest of our VT100 emulation, none of these features are implemented by interpreting escape sequences. The original VT100 Firmware already contains all the code required to support the AVO, so all PCjs has to do is emulate the additional hardware.

When the firmware starts, it reads the VT100’s “flags buffer” (port 0x42), and if bit 1 is clear, then it knows the AVO is present. The power-on memory test is then extended from the basic VT100’s 3K of RAM (0x2000-0x2BFF) to include the AVO’s RAM:

• 0x2C00-0x2FFF: 1K of additional screen RAM
• 0x3000-0x3FFF: 4K of attribute RAM

The attribute RAM is only 4 bits wide (the firmware’s memory test ignores the upper 4 bits of every location above 0x3000), and it parallels the VT100’s entire 4K of scratch and screen RAM, so the attributes for any character on the screen are always located exactly 0x1000 bytes above the character itself. As DEC’s VT100 Technical Manual puts it, “the AVO widens the entire screen RAM by 4 bits to make each character location 12 bits wide.”

The attribute bits are all “active low”, so the firmware initializes all the attribute RAM to 0xFF:

A VT100 without the AVO has only one attribute bit per character: bit 7 of the character itself, known as the “base attribute”. The VT100’s DC012 video control chip can be programmed to display that attribute as either reverse video or underline, and without the AVO, that choice is determined by the cursor (block or underline) selected in SET-UP, because the cursor is displayed by periodically toggling the base attribute of the character under it. This is why a basic VT100 with an underline cursor can display underlined characters but not reversed characters, and vice versa.

With the AVO, the firmware always programs the DC012 to display the base attribute as reverse video, and an underline cursor is displayed by toggling the underline bit in attribute RAM instead. So all the attributes are available at the same time, regardless of cursor.

To support bold and blinking characters, the DC012 has two video outputs, which combine to produce four beam intensities: off, dim, normal, and bright. According to the Technical Manual, here’s how every combination of attributes is displayed, where “Reverse” is the exclusive-OR of a character’s reverse video attribute and the reverse screen setting:

So blinking characters never actually disappear: they alternate between their usual intensity and the next lower intensity, unless they’re also reversed, in which case they alternate between reverse video and normal video. The DC012 doesn’t have a blink timer, either; it has a “blink flip-flop” that the firmware toggles roughly once every second, producing a blink rate that’s about half that of the cursor.

Underline simply turns on every dot of a character’s ninth scan line, which is why an underlined character in reverse video has a dark underline.

Finally, the alternate character set bit tells the VT100 to use the alternate character generator ROM on the AVO board. That ROM socket was normally empty, in which case every dot of the character is turned on. The firmware sets that bit when a character set that it calls an “alternate ROM” set has been selected (using the escape sequences ESC ( 1 and ESC ( 2).

Enabling the AVO

The AVO can be added to any PCjs VT100 by setting the avo property to true, either in the machine’s configuration file (see the “video” device) or in the list of machines at the top of a page:

machines:
  - id: vt100
    type: vt100
    name: DEC VT100
    config: /machines/dec/vt100/vt100.json
    avo: true

You can also add ?avo=true to the URL of any page with a VT100; for example, here’s a VT100 with AVO connected to SDF.ORG.

In the interest of preserving the appearance of our VT100 without the AVO, which never has anything to display at bright intensity, the dim, normal, and bright intensities are all displayed as white unless the AVO is installed.

[GitHub Source]