In the 1970s several projects were undertaken internationally to develop digital, network-based, public information-retrieval systems. These systems were referred to as "videotex" systems. Britain had the Prestel system, in France there was Minitel. The Canadian instance of such a project was Telidon. Inasmuch as these systems intended to provide residential end-use access to information, they can be seen as precursors to the World-Wide-Web, although they were much simpler and limited in their capabilities.

The Telidon project ran from 1978 to 1985 under the direction of the CRC (Communications Research Centre), a Canadian governmental R&D lab. An objective of the project was to define the techniques, standards and protocols for efficient transmission of graphical presentations within the limits of the available and affordable technology of the day. This meant transmission speeds around 1200 bits/second, display resolutions of a couple hundred pixels, and a colour palette with a limited number of bits/pixel. The protocol was optimised to minimise the number of bytes transmitted. In the early/mid-80s, the Telidon protocol would become the basis for NAPLPS - the North American Presentation Level Protocol Standard.

Along the way, telecom companies became involved and produced some hardware. Presented here is one of the physical remnants of the Telidon project, a Microtel VTX-202 display terminal manufactured in 1981/82. Microtel was a telecommunications equipment manufacturer. The VTX-202 incorporates a colour video display, keyboard, protocol decoder and modem. The keyboard is unfortunately missing from the unit here.

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Protocol & Graphics Capabilities

At the time of writing, no detailed information about the Telidon communications protocol has been found on the web. There is little technical information about NAPLPS on the web as well, although the Byte magazine articles linked above describe the protocol and encoding in enough detail to be useful.

Experimentation shows the VTX-202 implementation coincides to some degree with the NAPLPS protocol. The unit here is early enough that it predates some aspects of NAPLPS. The Screen & Graphics Test screen shown presents some samples of NAPLPS protocol elements transmitted to the terminal (Screen & Graphics Test source).

With the boot screen drawn at power-up, the terminal is ready to accept and display ASCII characters. As per NAPLPS, the ASCII control code Shift-Out (0x0E) switches to the PDI code set (Picture Description Instruction) for drawing graphics, Shift-In (0x0F) switches back to ASCII.

The terminal responds to 7-bit NAPLPS encoding, 8-bit has not been checked. An invalid protocol sequence produces a flashing "X" in the lower left corner of the screen.

Protocol Commands

The VTX-202 implements NAPLPS Set-position, Lines, Arcs, Rectangles, Polygons and Set-Colour. It does not seem to respond to Non-Selective-Reset, PDI-Reset, Select-Color or Texture. Other commands have not been tested.

Screen Resolution

X/Y coordinate data is sent as 3 operand bytes - a signed 9 bits per coordinate. The screen size in pixels is X=256 (horizontal) by Y=200 (vertical). X=0,Y=0 is the lower left corner. X=255,Y=199 is the upper right corner. In the standard or default size, characters are 5*7 pixels in a screen of 40 columns by 20 rows.

Color Capabilities

The VTX-202 is capable of displaying 8 colors and 8 gray levels. Black and white are included in both the color range and the gray scale so there are a net 14 distinct screen shades: This is achieved through two coloring modes, set by a 2-byte sequence: Note this is inconsistent with NAPLPS: 0x3D is a Wait command in NAPLPS.

When in color mode, the NAPLPS Set-Color command 0x3C with a single operand byte determines the drawing color. This provides 2 bits each for red, green and blue, which would be 64 colours, however only the 3 MSBs of the encoding are interpreted, as evidenced by the Color Test screen shown (Color Test source).

In gray-scale mode, the 0x3C command with a single-value operand byte determines the gray level to draw. The 3 MSBs specify the level from black to white.

The schematic indicates the memory is organised for 4 bits/pixel, consistent with 14 screen shades.


The following data byte sequence draws a red rectangle.
Tilde "~" indicates hex byte value. "~0D~0A" is CRLF.

~0C		: clear screen (CTL-L)
VTX-202~0D~0A	: some text
~0E		: switch to G1 (PDI)
~24~40		: position drawing point to 0,0
~3C~52		: set color to red
~31~40~7C~40	: draw filled rectangle
~0F		: switch back to G0 (ASCII Text)


Communication with the terminal can be accomplished via either an RS-232 connection or the internal modem. A configuration jumper selects one of these paths (see IC31 below).

The DB25F connector on the rear is RS-232 DTE. Transmit and receive baud rates are jumper-selectable from 150, 300, 600, 1200, 2400 and 4800 bps. 8N1 format will work. DCD must be asserted for receipt of data.

The RJ modular jack on the rear connects to the internal modem. This has not been experimented with to date, it is presumably 1200 baud.

Technical Notes