Editing Tektronix 4010 (section)

==Technical description==

===Mechanical layout===
The 401x series physically consisted of a large CRT display sitting on top of a wheeled cart. The cart held most of the electronics in a vertical case at the back, which could be opened from the front to access various switches and jumpers, as well as providing access to the expansion cards.{{sfn|Tektronix|1974|pp=1-2, A-2}} Inside the enclosure, expansion cards were connected via the Tektronix proprietary "Minibus" system, using a 36-pin card connector with an 8-bit data bus.{{sfn|Tektronix|1974|p=D-8}} In addition to the communications card and various enhancements, an optional "Desk-Top Mounting Kit" allowed the CRT to be placed on a desk, while the electronics cart was connected to it using a cable up to {{convert|10|feet}} away.{{sfn|Tektronix|1974|p=E-1}} The overall build quality of the Tektronix 4010 was excellent, with gold-plated boards of high-quality epoxy fiberglass material. {{citation needed|date=May 2024}}

===Interfacing===
The 4010 used a Minibus card to handle communications with the host computer, and a wide variety of host interfaces were available. The 4014 terminal was normally shipped with the standard Communications Interface installed, offering an [[RS-232]] connection, although only the most important connector pins were supported. Setup configuration was handled entirely by [[jumper wire|electrical jumper]]s,{{sfn|Tektronix|1974|p=A-6}} so there was no way for the terminal to change these settings while connected. As an alternative to the Communications Interface, the TTY Interface allowed the terminal to be connected to a [[Digital current loop interface|20-mA current-loop]] [[teleprinter|teletype]] interface, which was still widely used by [[mainframe]] computers of that era. Direct interfaces using proprietary serial or parallel connections were also available for most mainframe systems.{{sfn|Tektronix|1974|p=E-1}}

===Text display===
In ''alpha mode'', the 4010 displayed 35 lines of 74 characters. The terminal was "dumb" by standards of the day, lacking various [[Block-oriented terminal|smart terminal]] features like addressable cursor positioning. The terminal also lacked any significant data buffering, and many slower operations could lead to data loss. For example, the carriage return took about 100 to 200 μs, and a screen clear operation much longer than that, on the order of a second. It was up to the host computer to delay further data to avoid it being lost during these refractory periods.{{sfn|Tektronix|1974|p=2-9}}

One peculiar feature was a second margin at the 35th character, allowing lines to be limited between the left side of the screen and that midpoint for margin 0, or that midpoint and the right side of the screen for margin 1. This was useful for mixing graphics and text, or displaying two columns of text.  Switching between the columns was accomplished by moving to the very last line in any given column and pressing line feed on the keyboard. The cursor would then reappear at the top of the next column. There was no attempt to limit drawing to within these borders, so it was up to the host software to ensure that lines remained within the margins by inserting CR/LF characters at appropriate points. Text lines writing to margin 0 would extend the full length of the screen if CR/LF was not sent to wrap before the 35th character, although any data in the margin 1 area on the same line, being written later, would be drawn on top.

The terminals also optionally supported a second character set via plug-in [[circuit board]]s, selecting between them with the ASCII {{keypress|SI}} and {{keypress|SO}} characters.{{sfn|Tektronix|1974|p=2-7}} This was required for the [[APL (programming language)|APL]] language, which used a large number of special characters.

===Graphics protocol===
The 4010 was not a raster display and thus had an effectively unlimited resolution, but the command decoding circuitry limited this to 1,024 by 1,024. Because the screen had a 4:3 geometry, only 780 points were visible vertically. The origin was in the lower left.{{sfn|Tektronix|1974|p=3-28}}

Encoding values from 0 to 1,023 required 10 bits; 2^10 = 1024. These values were encoded in ASCII, using 5 bits per character, and thus requiring two characters per value, or 4 characters for a complete X,Y coordinate. The encoding scheme was designed to make the coordinate characters safely sent over serial links by assigning each value from the set of [[ASCII#ASCII printable characters|ASCII printable characters]]. X values were assigned the 32 characters between 64 and 95 decimal, mostly the uppercase letters. The Y coordinates are given a similar range from 96 to 127, mostly the lowercase letters. To convert location to an ASCII character code, one added 64 to the X value, and 96 to the Y value. High-order bits for either were the same, from the range 32 to 63, mostly numerals and punctuation.{{sfn|Tektronix|1974|pp=C-1 - C-4}}

So the complete formula for calculating the points from the characters was:{{sfn|Tektronix|1974|p=3-27}}

    X = 32 x (high X character ASCII value - 32) + (low X character ASCII value - 64)
    Y = 32 x (high Y character ASCII value - 32) + (low Y character ASCII value - 96)

Although the manuals always showed the calculation for X before Y, and low-order character before high-order, the four characters actually had to be transmitted in the opposite order, starting with the high Y, then low Y, high X and finally low X.{{sfn|Tektronix|1974|p=3-27}} For instance, consider the coordinate (23, 142). The X coordinate falls within the range 0 to 31, so no shifting is required. Adding 23 to 64 gives 87, which is ASCII character {{keypress|W}}, and since no shift is required the "shift character" is {{keypress|space}}. For the Y coordinate 142, one would need to shift the number back to the range 0 to 31, which could be done by subtracting 128. Doing so leaves 14. Adding 14 to 96 to get the first character gives 110, or {{keypress|n}}. This requires shifting by 128, which is 4 x 32, so the shift character is the fifth in the sequence (the first is zero shift, the space), or {{keypress|$}}. Now the characters are ordered shift-Y, Y, shift-X, X, so the complete coordinate (23, 142) would be encoded as {{keypress|chain=|$n W}}.{{sfn|Tektronix|1974|p=3-27}}

Each of these four coordinate characters is stored in a buffer in the terminal, which holds them until the complete coordinate is received and then drawn. The drawing process is triggered by the reception of the low-X character, which the terminal looks for by waiting for a bit pattern that indicates it is in the proper decimal range. This allows a short-cut way to send points that share a Y coordinate by sending only the X coordinates, even just the low-X if the high-X has not changed. This can greatly reduce the total number of characters sent to the terminal if the programmer arranges the data to minimize changes in Y over a given set of coordinates, and even more if they group together points that change only in low-X and low-Y. The overall effect can roughly halve the amount of data sent to the terminal.{{sfn|Tektronix|1974|p=3-27}}

Graphics are drawn by entering ''graph mode'' by sending the ASCII Group Separator (GS) character ({{keypress|Control|Shift|M}}). After that every set of four characters (or less) received by the terminal are used to define an X,Y coordinate. The first four following the GS position the graphic cursor, every point after that draws a vector on the display. The system is returned to text mode (''alpha mode'' in their documents) using a number of commands, typically Unit Separator (US, {{keypress|Control|Shift|O}}), but a number of other sequences also have the same effect, including a {{keypress|Return}}.{{sfn|Tektronix|1974|p=3-27}}

Because the system draws vectors from point to point while in graphics mode, in order to draw separated lines the commands had to enter and exit graphics mode repeatedly. A single point is drawn by entering graph mode at the desired coordinate, drawing a zero length vector to the same coordinate.{{sfn|Tektronix|1974|p=3-26}}

===Graphics input===
For graphics input, the terminal used a pair of thumb wheels on the [[alphanumeric keyboard|keyboard]] to control the position of a [[cursor (user interface)|cursor]]. The cursor was displayed using a lower intensity of the electron beam that had insufficient energy to cause the storage system to trigger. The cursor was dynamically refreshed by the electronics of the terminal. The cursor was turned on with {{keypress|ESC}} ({{keypress|Control|Shift|K}}) (which also turned off graphic mode if it was on), and then {{keypress|SUB}}  ({{keypress|Control|Z}}). The position was sent back to the computer using the same X,Y encoding as the graphics commands. This could be done interactively by sending the {{keypress|ESC|SUB}} and then pressing a key on the keyboard, or immediately by the host sending {{keypress|ESC|ENQ}}.{{sfn|Tektronix|1974|pp=1-7, 1-8}}

===Changes in 4014 model===
The 4014 series had a number of minor changes and some more major improvements.

In alpha mode, the font could be scaled to produce a number of different line sizes. The original 4010-style 35 lines by 74 characters was the default, or could be selected specifically with {{keypress|Esc|8}}. {{keypress|Esc|9}} drew smaller glyphs to produce 38 lines of 81 characters, {{keypress|Esc|:}} for 58 by 121, and {{keypress|Esc|;}} for 64 by 133. All of these could be mixed on-screen.{{sfn|Tektronix|1974|p=3-24}}

In the 4010, the cursor and graphic crosshairs were interactive and used ''dark mode'' to move about the screen without writing to memory. This was accomplished by writing with less energy in the beam, just enough to be seen but not enough to store it. The 4014 added escape codes to allow the user to select this mode deliberately by sending any of the sequences from {{keypress|Esc|p}} through {{keypress|Esc|w}}. This was especially useful in graph mode, as it allowed the system to draw movable objects, although at the cost of having to continually refresh them over the serial link at about 30 times a second in order to avoid flicker.{{sfn|Tektronix|1974|p=3-25}}

This capability could be used, for instance, by drawing the outline of a gauge and its scale markers normally to cause them to store, and then drawing the needle interactively using dark mode. It could also be used to move the graphics cursor to a new location without having to exit and re-enter graphics mode, which previously was the only way to accomplish this.{{sfn|Tektronix|1974|pp=2-8, 2-9}} Sending {{keypress|Esc|h}} through {{keypress|Esc|o}} set the terminal to ''defocussed mode'' which drew at reduced intensity by making the beam slightly wider and spreading the displayed line over a wider area. Finally, {{keypress|Esc|`}} through {{keypress|Esc|g}} returned the terminal to the normal storage mode.{{sfn|Tektronix|1974|pp=2-8, 2-9}}

The 4014 made a change to the way graphics points were entered by adding an ''execution character'' which denoted that a particular coordinate was complete. This allowed, for instance, the X or Y coordinate to be changed without changing the previously stored location for the other. This was useful for drawing boxes or especially a series of lines like an axis, or drawing a point on the screen by sending in the same address as the one last stored or moved to using dark mode. Since the X and Y coordinates used separate characters, the terminal would still notice a sequence of coordinates being sent in the older 4010 format, and draw them as they arrived, providing backward compatibility.{{sfn|Tektronix|1974|p=F-4}}

With the Enhanced Graphic Module installed, an additional set of features were available. Primary among these was the addition of ''12-bit addressing'' which increased the resolution to 4096 by 4096, again with the upper section of the Y axis above 3120 being unseen. Any address could be sent in 12-bit mode simply by sending in an ''extra byte'' between the high and low Y characters, using the same character range as the low-order Y addresses. On a 4010 series terminal or 4014 without the Enhanced Graphic Module, this extra byte would then immediately be overwritten by the actual low-order address that arrived as the next character, and thus have no effect. With the Enhanced Graphic Module, the terminal would use bits 1 and 2 to add to the front of the normally 5-bit high-order X address, and bits 3 and 4 to add to the high-order Y address.{{sfn|Tektronix|1974|p=F-4}}

Another feature of the Enhanced Graphic Module was circuitry that periodically interrupted the beam as it was drawing a vector, allowing the creation of dashed lines. There were five patterns in total; lines, dots, dash-dot and short and long dashes. These were useful for drawing axes and scales, especially when combined with the defocussed mode to lower the intensity, and using the change-one-coordinate feature to quickly draw them. These were selected using the same escape characters as the drawing mode selection of the normal 4014, the range {{keypress|Esc|`}} through {{keypress|Esc|w}}. For instance, without the Enhanced Graphics installed, sending any character from {{keypress|,}} to {{keypress|d}} selected normal line drawing mode, while with the module installed {{keypress|,}} was normal drawing, {{keypress|a}} was normal with dotted lines, and so on.{{sfn|Tektronix|1974|p=F-6}}

Incremental plot, entered with the ASCII Record Separator (RS) character, replaced the normal coordinates with single-character directions. For instance, sending {{keypress|E}} moved up ("north"). This was especially useful for drawing control needles and similar moving displays, and greatly reduces the amount of information that has to be sent to the terminal over time.{{sfn|Tektronix|1974|p=F-7}}

The Enhanced Graphic Module introduced two [[point plotting]] modes. Entering the normal ''point plot'' mode with the ASCII File Separator (FS), instead of RS for graph mode, plotted only the points at the coordinates being sent, not the vectors between them. ''Special point plot'', entered with {{keypress|Esc|FS}}, added an ''intensity character'' to the coordinate that allowed the points to have various brightnesses and optionally defocus the beam.{{sfn|Tektronix|1974|p=F-8}}