MIT Lincoln Laboratory · TX-2 reconstruction

Sketchpad

The reconstructed 1963 TX-2 assembly executes inside your browser.

OSCILLOSCOPE DISPLAY TIMED POINT BEAM · UNIT 60

STATE INITIALIZING

TX-2 TIME 0.000000 s

POINTS 0

LIGHT PEN UP

SELECTION NONE

INPUT HANDLER WAIT

PEN APPLY WAIT

LIGHT-PEN HARDWARE PICKUP RADIUS 12 UNITS

This model maps sensitivity directly to the light-pen detection radius.

SHAFT ENCODERS Q4 000 Q3 000 Q2 000 Q1 000 KNOB META

SKETCHPAD TOGGLE REGISTERS

These switches change TX-2 hardware state. The assembly reads them directly.

DRAW CYCLE377720 · 4.9 · DRAWASFIX SOLVE377720 · 4.10 · FIX SELECTABLE RECORDS377725 · 4.9 · SHOWBLKS CONSTRAINT MARKS377725 · 4.8 · SHOWCON POINTS377725 · 4.7 · SHOWPOINTS CONSTRAINT HANDLES377725 · 3.9 · SHOWTPVALS SUPPRESS LINES377725 · 4.4 · SUPPLINES CONSTRAINT LETTER377725 · Q1

EXTERNAL INPUT · 377621

Hold one or more buttons. Hover a named bit to see the recovered assembly routine.

LOAD PAPER TAPE

VISUAL DIAGNOSTIC LOG

This records the scope plus a cyan input cursor and state label. The overlay exists only in the diagnostic video. The recorder stores one-second media chunks. Stop it before you download one intact WebM file.

READY

Loading the WebAssembly machine.

Click once to engage the pen. Move across INK to acquire its first position. Press Escape to disengage it.

Operator guide

Use the real Sketchpad input path

Position the pen. Move the pointer anywhere over the scope. The browser continuously sends that position to the emulated light-pen hardware.
Acquire the pen. Click once to engage the sensor. Then move across the bright INK letters. You do not need to click again. The original photocell cannot detect blank glass. Light pen says ACQUIRING until Sketchpad stores the first real pen position. Wait for TRACKING. Press Escape when you want to disengage the sensor.
Draw. Hold D to send Q1.8 STARTDRAW. Move the tracker without holding the mouse button. Release D. The browser then holds Q1.6 STOPMOVEP. The original assembly finalizes the line. The next external-input command releases Q1.6.
Select. Move over the middle of a visible line. Click once if the sensor is disengaged. Wait until Selection says LINE. Send the next command while the sensor remains engaged.
True up and solve. Leave Solve off. Tap T once. This sends Q2.9 to TRUEUP. The assembly adds one horizontal-or-vertical constraint. Then turn on Solve. The original RELAX routine changes the endpoints.
Fix and release. Select an object. Tap F once. FIXIT puts it in the original fixed-object list. U runs UNFIX and clears the fixed and designated lists.
Record a fault. Tap R to start the visual log. Tap R again to stop it. Then use the download button when the drawing is complete.

Make adjacent edges mutually perpendicular

Draw one closed rough flange. Connect each new line to the bright endpoint of the prior line.
Select P · 037. Turn on Points and Constraint handles.
Aim at blank glass. Hold C until Q2.8 MAKECONS makes one P constraint. Hold S to place it.
Hold H over one visible P handle. Hold M until Q2.1 MOVEPOINT starts it moving.
Keep H held. Move the handle onto one endpoint. Hold S until Q1.6 merges it with that point.
Attach all four handles to the two endpoints of two adjacent edges. Make one P constraint for each adjacent edge pair.
If many handles slow the display, turn on Suppress lines while you attach them. Turn lines on again before the solve.
Turn on Solve. Leave it on for several display cycles. The original assembly repeats RELAX until the shape settles.

The P constraint chooses the nearer parallel or perpendicular solution. Start with each adjacent pair visibly closer to perpendicular.

Unit 55 reports only an optical hit. It does not report coordinates. The page shows LOST only after LPLOST remains set for 150 milliseconds. This prevents the internal tracker search from making the readout flash. The browser still sends every physical D-button transition. A fast jump can outrun the original tracker search pattern. Move back across a bright tracker mark to reacquire it. A larger Pickup radius models a more permissive detector.

The short perpendicular stroke at a moving endpoint can be the original tracker search pattern. Use Visual diagnostic log when a discontinuity does not look like that pattern. Stop the recorder, download the WebM file, and attach it to the project report.

Input handler measures browser work from one pointer event to publication of the physical pen state. Pen apply measures transfer of that state into the worker-owned emulator. The visible tracking marks still follow the original assembly and scope timing.

The Q4-Q1 panel remains available for every external-input bit. Named buttons come from the recovered READIT dispatch table. Keyboard shortcuts only press those emulated physical buttons. They do not create or edit geometry in JavaScript.

Advanced command status

Input	Routine	Status
Q1.8 · `D` down	`STARTDRAW`	Verified line creation
Q1.6 · `S`	`STOPMOVEP`	Verified moving-object completion and point merge
Q2.9 · `T`	`TRUEUP`	Verified horizontal-or-vertical constraint
Q3.3 · `F`	`FIXIT`	Verified fixed-set link
Q2.7 · `U`	`UNFIX`	Verified fixed-set removal
Q1.7	`DESIGNATE`	Verified designation and circle-arc creation
Q2.1 · `M`	`MOVEPOINT`	Verified P-variable movement
Q2.8 · `C`	`MAKECONS`	Verified P-constraint creation
Q4.9 · `H`	`HOLD`	Verified movement through transient pen loss
Q2.6	`MAKPATA`	Recovered route. Attachment workflow is not yet verified.
Q2.4	`SUBPIC`	Recovered route. Instance workflow is not yet verified.

Q2.8 reads the selected constraint letter from toggle register 25. It does not infer a type from the aimed-at object.