green-green-avk / vt340test

Tests of VT340 compatibility

What is this?

In the decades since the DEC VT340 terminal was released, knowledge of how it functioned has been lost. Mostly for archaeological purposes, but also so that the lsix program works correctly, hackerb9 purchased a VT340+ and is running tests, many of which were submitted by j4james, and documenting the results here.

Test files and output

• Please see the j4james directory for test files and their output.

• See sixeltests for a few sixel test images and scripts.

Notes on Hardware VT340

• DECSDM (Sixel Display Mode), when enabled, DISABLES Sixel Scrolling in the Graphics Set-up screen and vice versa.

• Showing images with 16 colors messes up text foreground/background.

  • Soft Terminal Reset does not reset color palette.

    CSI ! p
    echo $'\x9b!p'	 # or $'\e[!p'
    

  • Hard Terminal Reset does reset palette, but it takes a long time to execute as it goes through the whole power-on self-test again. echo $'\ec'

  • It is difficult to reset colors from the builtin setup because the current color map is used for the color setup screen so you can't see what you're doing.

  • To reset colors from keyboard (without needing to look):

    [Set-Up] [Prev Screen] [Do] [Set-Up]
    

    That pulls up the palette editor and recalls the saved color map.

  • Switching to Tektronix mode and back does not restore the palette.

    echo $'\e[?38h\e[?38l'		# Colors do not change
    

  • Unlike REGIS, sixel color numbering is different from VT340's setup screen numbering! No matter what number you assign a sixel color, it is only the order that you did the assignment that matters in terms of where it is put in the VT340 color map. Strangely, the first sixel color defined is mapped to VT color #1, not color #0. The sixth color assigned (color #7) becomes the foreground color. The sixteenth assigned color wraps around and modifies VT color #0, so to set the background color, one must set all the other colors first. This is probably intentional: a sixel file that doesn't make use of all 16 colors would not make annoying changes to the background.

  • To reset the color palette programmatically, try hackerb9's resetpalette.sh script which uses DECRSTS (Reset Terminal State) to set the color table directly instead of using sixel commands. It uses the actual Color Table Report returned by his VT340+ in response to DECRQTSR (Request Terminal State Report).

  • OPEN QUESTION: How does one convert an image to sixteen colors, but with three of the colors (fg, bg, and bright) fixed and the others free?

  • Note: lsix splits the montage into rows to reduce waiting when there are more than 21 images to show. However, each montage has separate color map, which means the previous row's colors will get messed up. I'm not sure there's a good solution for this other than to force a fixed palette (would grayscale be a good idea?).

Number of colors on a genuine VT340

• A genuine VT340 can show 16 colors from a 12-bit palette, which means there are 4096 (2^12) different colors available. ImageMagick calls this -depth 4, because there are 4-bits per color channel.

• Red, Green, Blue vary from 0 to 100% intensity.

  RGB can specify 101×101×101 different colors (nearly 2^20).

• Hue varies from 0 to 360 degrees, Lightness and Saturation vary from 0 to 100%. (Hue 0 and 360 are identical).

  HLS can specify 360×101×101 different colors (over 2^21).

• Thus there are roughly 250 different RGB values and 1000 different HLS values for each possible color that can actually be shown.

• When quantizing colors (reducing the color palette to 16) using ImageMagick, it may help to specify -depth 4 so ImageMagick doesn't allocate two colors that are functionally identical.

VT340 Screen Resolution

Graphics screen resolution is 800x480, but sometimes is quoted as 800x500, which includes the 25th line (the status line).

• The status line is only addressable by sending a special escape sequence (DECSSDT, DECSASD):

  tput tsl; tput rev; echo; echo -n $(date); tput fsl)
  

  Even if one wanted to muck with that, the status line ignores sixel commands, so it is still not possible to use it for graphics.

REGIS

• To enter REGIS: send from host ESC P 3 p

• To exit REGIS: send from host ESC \ or hit Control-L

• Example of using REGIS

    clear
    echo -n $'\eP3p'        # Enter interactive REGIS mode
    echo -n $'P[150,200]W(S1)C(W(I1))[+100]C[+66]C(W(I2))[+33]' # RAF roundels
    echo -n $'\e\\'     # Exit REGIS mode
  

MEDIA COPY TO HOST ("screenshot"):

Media Copy tells the VT340 to transmit a sixel copy of the current screen to the host.

See:

• mediacopy/README.md: Investigations of how Media Copy works on VT340.
• mediacopy/mediacopy.sh: script that saves a screenshot in "print.six".

At the moment, it is required to use the VT340's Printer Set-Up to manually change the Sixel Graphics Mode to "Level 2" before using media copy. Without that, it sends Level 1 output with pixel aspect ratio set to 2:1.

The script isn't working quite right yet because occasionally the VT340 pauses transmission in the middle and causes an 8-bit glitch in the output data. (Could just be hackerb9's terminal or his serial port connection?)

Keyboard

• The ESC key on the LK-201 keyboard only functions in VT100 mode. Instead, you must use ^[ or ^3.

• Despite what the manual says, you cannot use "Return" to select elements in Setup. You must use the "Do" key.

• The LK-201's Shift does not affect , and .. Instead the < and > symbols are on their own key next to the left shift. This is not changeable from the VT340 Set-Up menu.

Tek 4010/4014 mode

• Entering and exiting this mode leaves VT340 with autowrap mode turned off. This appears to be a firmware bug.
  • To test: echo $'\e[?38h\e[?38l'
  • To fix, set DECAWM after exiting Tek mode: echo $'\e[?7h'
• Sixel print to host appears buggy; last portion gets sent to screen instead of to the host.

80/132 column mode

• Despite the documentation recommending DECSCPP (set columns per page) and deprecating DECCOLM (column mode) to switch between 80 and 132 column mode, they do not appear to be equivalent.

  • DECSCPP: Esc [ 8``0 $ | or Esc [ 1``3``2 $ |

    Terminal now thinks it has that many columns, but the screen font doesn't actually change. Has the benefit of not clearing data in page memory or changing number of lines per page. If you switch to 132 columns, but are using an 80 column font, then text that is off the screen simply isn't shown. It is unclear how this is beneficial.

  • DECCOLM: Esc [ ? 3 l or Esc [ ? 3 h

    Terminal switch to 80- or 132-column mode, the same as if it had been changed in the Set-Up -> Display screen. Both the logical width and the actual font size change. Resets page memory.

Dual sessions

The VT340 allows two login sessions simultaneously. While it is possible to do over a single serial cable using DEC's proprietary SSU protocol, it is easier to just use the VT340's two communication ports. (See flowcontrol for how to build a DEC423 cable for Comm2).

Press F4 to switch which session is active. Use Ctrl+F4 to split the screen vertically or horizontally.

Sixel images persist on completely separate framebuffers. While each session can have a separate color palette, the VT340 hardware can only use one at a time. That means split screen images will only look correct for the session that is currently active.

Note that when split horizontally, the VT340 attempts to scroll the view up or down to where the cursor is. This works well most of the time, but full-screen applications can have problems. For example, a text editor that has the text cursor typing at the top of the screen, but is also updating a status line at the bottom that shows the current column and row would jitter up and down rapidly on each keystroke.

Printer port

The printer port is actually a third serial port, with the same DEC423 wiring as Comm1 and 2. As you'd expect, a print command, such as Esc [ i, sends data to it. What is unusual is that any data received on the printer port is typed into the active session as if it was from the keyboard.

Although it defaults to 4800 baud, the same as the actual keyboard port, it can be set to run at 19.2Kbps.

This could be useful in the future for automating compatibility tests by connecting the printer port to a host computer. [Hackerb9 currently has Comm1 and 2 on /dev/ttyS0 and S1 and the printer on /dev/ttyUSB0.]

Note that DEC's printer port is necessarily bidirectional because the VT340 needs to listen for XON/XOFF so it can pause when the printer is being overrun with data.

Pages and Lines per page

The VT340 can store several "pages" of text in memory. Is "Page Memory" like the scrollback buffer we are familiar with on modern terminal emulators? It does not seem so.

Pages appears to be intended for application use, perhaps for some sort of task switching where you'd want to get back to a previous screen without having to resend all the data.

• There are sequences, NP (Esc``[``U) and PP (Esc``[``V), which can scroll forward and back in "pages", but it appears the pages are only written to when an application specifically addresses them. When data scrolls off the top, it doesn't get put into a page.

• Page Memory can be used like double-buffering, where text is written to the next page while the user is viewing the current one. (See EK-VT3XX-TP.pdf#Ch10).

• Manual mentions that there are two sixel graphics pages (unless you are logged in to two sessions at once). It appears they are addressed the same way as standard page memory, but only the first two pages actually work. This means graphics double-buffering is possible!

• Future questions

  • Did/do any programs actually use Page Memory?

  • Is there an example of using the graphics pages for double buffering? (For example, for decoding animated GIF images.)

  • Is there a quick way to have the VT340 copy one graphics page in memory to another? For example, in an animated GIF, only the first frame should have to be sent, after that the differences for subsequent frames take up much less bandwidth. Without a quick way to copy pages, double-buffering would mean the first two frames would have to be sent and deltas would have to be applied twice.

  • If the VT340 had more RAM, could it have held sixel bitmaps associated with each of the text pages? If so, how hard would it be nowadays to upgrade the hardware/firmware to do this?

  • DECSLPP lets one set the VT340 to have more "lines per page" and concomittantly fewer "pages", or vice versa, but what good is that if the pages are too big to display on the screen? Is it fast to pan pages?

XON/XOFF Flow Control is Required

It does not appear that the VT340 firmware can perform RTS/CTS flow control, although electrically it is possible. See flowcontrol for more details.

Smooth scrolling is on by factory default

Instead of scrolling the page up as fast as possible ("Jump" scrolling), the VT340 defaults to "Smooth-2" scrolling. Smooth-1, Smooth-2, and Smooth-4 are the three possible smooth scrolling speeds available in Set-Up -> Display -> Scrolling.

On the VT340, this setting does not affect sixel images when first being drawn. Graphics New Line ('-') at the end of the screen still scrolls the entire screen as fast as possible. However, once on the screen, sixel images do scroll smoothly along with the text upon receiving a text New Line ('\n').

Documentation varied about how fast the different speeds were supposed to be, so hackerb9 measured the speeds using the scrollspeed.sh shell script.

Results:

Setting	Scanlines per second	Text lines per second
Smooth-1	30	3
Smooth-2	60	6
Smooth-4	119	12
Jump	596	60
No Scroll	N/A	N/A

While the VT340 allows the scrolling speed to be changed in the Set-Up menu, it does not appear to be programmatically changeable (as it is on the VT5x0 using DECSSCLS, Set Smooth Scroll Speed). Instead, DEC Private Mode #4, DECSCLM, Smooth Scroll Mode, is used as a binary switch. When DECSCLM is SET, Smooth-2 is selected. When DECSCLM is RESET, Jump scroll is used. Querying the private mode via DECRQM returns SET when any of the Smooth speeds are selected. If the user selects "No scroll" in the Set-Up menu, then DECRQM returns NOT RECOGNIZED.

Note that, although Smooth-2 is the factory default on the VT340, the most popular terminfo file for the VT340 (as of 2023) disables Smooth Scroll Mode when the "reset" sequence is sent, which is often done at user login (e.g., tset). This makes having a user preference of Smooth-1 or Smooth-4 overly onerous as they would require repeated manual configuration in Set-Up.

Character Sets

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It appears the easiest way to use a VT340 in modern times is to enable the Latin1 character set in Set Up and and export LANG=en_US.iso8859-1. However, there may be ways to get more glyphs out of the VT340 without even down-line loading a new font. Please see the charset subdirectory.