vt1200/reference/keyboard-mouse/elkay/docs/phase1-bench.md

Phase 1: characterising the keyboard

Goal: before building anything, find which connector pin is which, how much current the keyboard draws, and what voltages its data line uses. Only a multimeter and a 12 V supply are needed; no microcontroller yet.

Record every result in the table at the end and commit it.

What you need

• LK401. Its cable is hardwired at the keyboard and ends in a 4P4C (RJ10) plug, about 7.7 mm wide. Don't cut it.
• Multimeter
• USB adjustable power module with a display (3 W, short-circuit protected) as the bench supply. The MT3608 is kept for the final box.
• An RJ10 (4P4C) female-to-female coupler, 1:1 wiring
• An RJ10 plug-to-plug flat cable with one end cut off and the four wires stripped
• A breadboard or terminal block for the flat cable's wires; jumper wires
• A 4P4C jack breakout board with screw terminals. They are often sold as "RJ9"; RJ9, RJ10 and RJ22 all mean the same 7.7 mm, 4-contact handset jack. Not RJ11 or 6P6C, which is wider.

The first setup is: LK401 plug → coupler → flat cable → four bare wires. The flat cable's wires are fine and break easily, so once the breakout board has been mapped (section 2) the LK401 plugs straight into it instead, and everything connects to its screw terminals.

1. Set the supply

1. Plug the power module into USB, ideally on an extension cable, with nothing connected to its screw terminal output except the multimeter.
2. Find the + terminal: a positive reading means the red probe is on +.
3. Turn the trimmer (clockwise raises it) until the multimeter reads 12.0 V. The module's own display can be a few tenths out. Too much voltage can damage the keyboard.
4. Unplug it again.
5. Before the first power-up, do the load test. Put a 1 kΩ resistor across the output for 5 minutes. If it holds steady, re-set it to 12.0 V with the resistor in place. If it keeps climbing, don't use it on the keyboard until that is understood.

A multimeter with a weak battery can read high and drift. An apparent upward creep of about 0.8 V turned out to be exactly that. If readings wander, change the meter battery first.

2. Map the flat cable (no keyboard needed)

This finds which bare wire goes to which pin, so the colours found in section 3 can be checked against the pin order in lkkbd.c. The keyboard is not involved.

Pins are numbered as in protocol.md: hold the plug with the cable downwards, the latch away from you and the contacts facing you. Pins 1–4 then run from left to right.

1. Meter on continuity (beeper), or the lowest Ω range.
2. For each plug contact 1–4 in turn, hold a probe tip on the contact's gold edge and touch the other probe to each bare wire until it beeps. Record the colour. Each contact should connect to exactly one wire, reading under about 1 Ω.
3. Check for shorts: probe each of the six pairs of bare wires. All should read OL.

Once the coupler has arrived, map through it too, before the keyboard is plugged in:

4. Plug the flat cable into one side of the coupler. Look into the empty side with the latch slot at the top. The contacts are along the bottom, and pin 1 is on the right, because a socket is the mirror image of a plug. The keyboard's pin 1 will land on that contact.
5. Meter probes are too thick for the socket, so hold a jumper wire's male pin against one probe and use it as a fine tip. Touch each socket contact and find its bare wire, as in step 2. Record the colours. This maps each keyboard pin to a colour, and shows whether the coupler is straight or crossed.

Once the breakout board has arrived, map it before the keyboard goes near it. Boards label their terminals differently, and some number from the other end, so don't trust the labels:

6. Plug the flat cable's plug into the board's jack.
7. On continuity, find which screw terminal beeps with each colour. Using the keyboard pins from step 5, label each terminal with its pin number and signal.
8. Plug the LK401 into the board and repeat the pin 1 check from section 3, step 4, on the 20 MΩ range. If pin 1 still reads open, the coupler's contact is ruled out as the cause.

3. Find ground and +12 V (keyboard unpowered)

The keyboard is not opened and its cable is not touched: its plug sits in the coupler exactly as it would in a terminal. Everything is measured on the cut end of our own flat cable.

1. Plug the LK401's plug into the coupler and the flat cable into its other side. Push the four bare wires into a breadboard or terminal block so none can touch. Label each wire with its colour and the keyboard pin section 2 mapped it to.
2. Per lkkbd.c the order is 1 RX-in, 2 GND, 3 +12 V, 4 TX-out. A reversed cable only flips it end to end, so the two middle wires are the supply and the two outer wires are data. Check this in the next steps rather than trusting it.
3. Meter in diode-test mode. It drives about 1 mA at 2–3 V, harmless to the keyboard. Measure between the two middle wires, then swap the probes:
   • red on +12 V, black on GND: the reading climbs and ends at OL, as the meter charges the keyboard's supply capacitor;
   • red on GND, black on +12 V: a steady 0.4–0.7 V through the protection diodes of the keyboard's chips.
4. Red probe on GND, black probe on each outer wire in turn. The keyboard's output (pin 4) should show a diode drop. Its input (pin 1) may read OL: on our keyboard it read OL to every other pin in both directions, in diode test and on the 20 MΩ range. Whether that is a high-impedance input or an open wire is settled when the keyboard is first sent a command (verify). A reading of 0 on any data wire is a short: stop.
5. If the middle wires don't behave as in step 3, stop and record every pair's readings in both directions before powering anything.

4. Power up and measure current

1. Put the multimeter in DC current (mA) mode, in series between the supply's +12 V output and the +12 V pin. Connect the supply's ground to the GND pin.
2. Power up. The keyboard should beep or click and flash its four LEDs, which is its self-test.
3. Record the current at idle, the peak as the LEDs flash, and the current with keys held.
4. If the supply's display blanks or the keyboard keeps restarting, the supply is hitting its 3 W limit (about 250 mA at 12 V). Note it; the final MT3608 has more headroom.

5. Identify the data pins and their levels

Keyboard powered, multimeter in DC volts, black probe on GND:

1. Measure each of the two data pins at idle. The keyboard's output (TX) is driven, so it will show a steady voltage. Its input (RX) will read near 0 V or float.
2. On the TX pin, hold a key with auto-repeat, such as a letter, and watch the reading. A multimeter can't show the serial data, but the average voltage will shift while bytes are being sent.
3. Record the idle voltage of TX. It tells us:
   • idle near 0 V, active high: RS-232 polarity, and the receive path needs an inverter (the expected case);
   • idle near +12 V or +5 V: TTL polarity, and a resistor divider is enough.

Results

Measurement	Value
Supply display offset	display 12.4 V = meter 12.0 V (1 kΩ load)
Supply drift, 1 kΩ load	holds 12.0 V (step 5 passed)
Flat cable plug, pins 1–4 (colours)	1 yellow, 2 green, 3 red, 4 black; no shorts between wires
Through coupler, keyboard pins 1–4 (colours)	1 yellow, 2 green, 3 red, 4 black
Coupler wiring (straight / crossed)	straight
Pin 1 (yellow) resistance	OL to every other pin, both directions, on 20 MΩ
Breakout board, terminal for pins 1–4
Pin 1 resistance via breakout board
GND wire (colour)	green (keyboard pin 2)
+12 V wire	red (keyboard pin 3)
Keyboard TX wire
Keyboard RX wire
Current, idle
Current, LED flash peak
Current, keys held
TX idle voltage
TX voltage with a key held
Diode test, +12 V → GND (red → black)	OL
Diode test, GND → +12 V	771 mV
Diode test, GND → each data wire	yellow (pin 1) OL; black (pin 4) 1027 mV

With these, the interface circuit in hardware/README.md can be confirmed and the power budget checked: USB 2.0 gives 500 mA at 5 V, and the boost module turns roughly 1 mA at 12 V into 3 mA at 5 V.