TouchDRO TDK-40/T Adapter KIT Manual

TouchDRO TDK adapters will be available for pre-order in a few days, and shipping will start in the first week of February.

TouchDRO TDK-40 is an easy to build 4-axis DRO adapter kit that supports RS-422 (differential) and TTL (single-ended) glass and magnetic DRO scales and other 5V quadrature encoders. It uses the same powerful 32-bit architecture and differential input circuits as the TDA-4xx adapters, and offers the same speed and performance.

TouchDRO TDK-40 DIY Adapter Kit (Front)

The kit can be ordered in two configurations:


TDK-40 is the base model that supports up to four RS-422 or TTL scale inputs and is designed to work with most modern optical and magnetic DRO scales.

Electonic parts included in the TDA-40 DRO Adapter Kit
3D printed enclosure parts included in kit


The TDK-40T is similar to the TDK-40, but adds support for a tachometer input. The kit includes two additional parts:

TouchDRO TDK-40T includes a 4-pin Mini-DIN tachometer connector


Four RS-422/TTL Scale Inputs

The adapter comes with four scale inputs that support 5 Volt RS-422 and TTL quadrature encoders. This includes the vast majority of modern DRO scales (optical, magnetic and inductive) and NPN, PNP, and Push/Pull incremental rotary encoders.

TTL scales (also called single-ended) use two lines, A and B, to send the position data to the DRO. RS-422 inputs (also called differential) use a complementary pair for each line. The additional lines are often marked as A'/B', A-/B-, etc. Those lines carry inverted signals, so when line A is high, line A' is low, and vice versa. When the two inputs are combined by a special differential driver circuit, the noise is canceled.

RS-422 is good to have but not strictly necessary, unless the scale cables are very long (multiple tens of meters).

Bluetooth Connectivity with USB Mirroring

TouchDRO TDK adapters are equipped with a BlueTooth 4.2 transceiver and a high speed USB bridge. The firmware uses BlueTooth as the primary connection and mirrors the position output to USB, which can be used for real-time data capture, etc.

Please note that USB outputs position data only when BlueTooth connection is active.

Tachometer Input

TouchDRO TDK-40T adapter model supports a tachometer input that can be used with any NPN or Push/Pull optical encoders or HALL effect sensors to measure the spindle RPM. The tachometer input can handle input frequencies ranging from 0.5 Hz and 100KHz with measurement error of less than 1%.

Compatible DRO Scales

The adapters support DRO scales and rotary encoders that output 5V quadrature signals. This includes the vast majority of modern optical, magnetic, and inductive scales. Learn More

Technical Characteristics

Position Refresh Rate

The adapters send the scale position to the application every 40 milliseconds (25 times per second) when active, and once per second when idle.

Quadrature Signal Frequency

The maximum recommended quadrature signal frequency is 1 MHz. This is equivalent to a 1 micron scale moving at 4 meters per second, or a 5 micron scale moving 20 meters per second. The absolute maximum frequency is a bit under 4 MHz.

The input circuit can reliably handle 32MHz signal, and the main CPU will not start losing pulses until about 160 MHz, but in order to better handle noise, the firmware uses glitch rejection mechanism that will ignore any pulses that are less than 250 nanoseconds (per line). If you have a special application where 4 MHz is not fast enough, the firmware can be configured with lower glitch rejection value.

Tachometer Signal Frequency

The adapters have a low limit of 0.5Hz and upper limit of 100 KHz input frequency for the tachometer. In other words, the adapter will count pulses that are at most 2 seconds apart, and up to 1/100000 of a second. In terms of RPM, this translated to 30 RPM minimum and 6 million RPM maximum with a single-slit tachometer disk, or 3 RPM to 600,000 RPM with a ten-slit disk.

Minimum/Maximum Ratings

  Nominal Minimum Maximum
Supply Voltage 5 V 4.0 V 5.5 V
Current Draw 15 0ma 50 ma 300 ma
Primary Scale Inputs
Signal Frequency 1 MHz --- 4 MHz
Min Edge Separation 250 ns 250 ns ---
Signal High Level 5 V 2 V 5.5 V
Signal Low Level 0 V -0.3 V 0.8 V
Tachometer Input
Tachometer Signal Frequency 100 KHz 0.5 Hz 100 KHz
Signal High Level 5 V 2 V 5.5 V
Signal Low Level 0 V 0 V 1.1 V

Assembly Instructions

When you receive the kit, it will be partially assembled:

Required Tools and Materials

You will need the following tools and materials:

Build Instructions

Step 1 - Install the ESP32 Module

Remove the ESP32 module from its antistatic bag and insert it into the main board, as shown below.

ESP32 Module inserted into the DRO carrier board.

Press the module against the board and solder two diagonally opposed leads to hold the module in place and check that the pin header is sitting flush; adjust if needed.

Carefully solder the remaining leads. Make sure that the molten solder is wetting the pads and the leads, and creates a concave filet.

Now, it's a good time to power up the module from a compatible USB power supply and check that the green LED is lit and blue LED is blinking. The Red LED on the module will be lit; this indicates that the module is receiving power. The Green LED indicates that the main board is receiving power, and the blinking Blue LED shows that the TouchDRO firmware is running.

Step 2 - Install the Pin Headers

Insert the first pin header into its position and solder one of the pins while pressing the header by an opposite pin. Do not touch the pin you are soldering; you will get burned.

Repeat for the other 3 pin headers.

Lay the board face-down on a flat surface, press it down, and one-by-one briefly re-melt the soldered pins. This will straighten the connectors.

Solder the remaining pins, making sure that you don't create solder bridges between adjacent solder joints. The end result will look similar to the photo below.

Finished TouchDRO TDK-40 DIY adapter kit

Step 3 - Install the Mini-DIN Connector

This step applies only to the TDK-40T kit.

Insert the Mini-DIN 4 connector into the main board and solder the pins as shown below.

TDK-40T kit includes an additional 4-pin Mini-DIN connector

Step 4 - Clip the Protruding Pins

The pins on the underside of the board will protrude by a few millimeters. Using a pair of clippers, cut the pins so they protrude by no more than 2 mm.

Pins being clipped to under 2mm.

Step 5 - Built the DB9 Connectors

Determine the Scale Pinout

Locate the pinout diagram for your scales and determine which pins will be used to connect it to the TouchDRO adapter. This information is usually included with the new scales, or can be obtained from the scale supplier. Alternatively, if you can't locate the relevant documentation, check the "Supported Optical and Magnetic DRO Scales" to see if your scales use one of the standard pinouts. Finally, you might be able to reverse engineer the pinout using the "How to Find Glass DRO Scale Pin Functions" guide.

Depending on the type of your scales, you will use four or six wires per connector:

Prepare the Wires

The kit comes with 12 Dupont jumper ribbon cables.

Prepared D-Sub 9 connector wires

Cut the provided heat shrink tubing piece in half, and slide the resulting pieces over the wires you just prepared.

Solder the Wires

For the next operations it's best to hold the connector body in a small vice or a set of "helping hands", as shown below.

Build one connector according to your scale's pinout, but don't seal the heat shrink tubing. To ensure that the pinout is correct, plug in the connector into the board, and carefully plug in the scale making sure that the exposed wires are not touching anything conductive.

Power up the adapter, connect to it from the TouchDRO application and confirm that the readouts change when you move the reading head:

If everything looks OK, solder the remaining three connectors.

Move the shrink tubing over the soldered pins on the DB9 connector and using a heat gun or a butane lighter, carefully heat it until it shrinks uniformly around the pins. Finished connectors should look like the photo below.

Finished DB9 connectors

Step 6 - Attach DB9 Connectors to the Face Plate

Using two 4-40 jack nuts and two 4-40 nuts, attach each of the DB9 connectors to the side plates.
The end result should look like the photo below.

The connector plates with attached DB-9 connectors

Step 7 - Assemble the Enclosure Bottom

Place the PCB into the base of the enclosure. Use two included black "Plastite" screws to attach the front side to the base. Check that the Micro-USB connector is aligned with the opening and the edge of the module is flush against the plastic enclosure side.

Front side sits flush against the ESP32 module

Attach the back side to the base.

Inset the DB9 connector plates into the slits as shown below and plug in the wires into the headers according to your scales pinout. Refer to the TDK-40 pinout diagram above if needed.

Note that the connector plates will fit tightly into the sides.

The connector plates inserted into the body

Step 8 - Assemble the Enclosure Lid

Remove the protective film from the acrylic insert and place it into the cutout in the enclosure lid. Note that the TouchDRO logo should be facing towards you.

The connector plates inserted into the body

Step 9 - Test the Adapter

Before closing the lid, it's a good idea to test the scale inputs to make sure that they are wired correctly.
Plug in the scales into the adapter.

Power up the adapter from a compatible USB power supply and pair it with the TouchDRO application. Refer to the Connecting To TouchDRO Scale Adapter page if needed.

Perform the initial configuration as described in the TouchDRO Initial Configuration guide. Once the application is configured, move the scale and confirm that the readouts respond to the movement.

If everything looks good, use the remaining four Plastite screws to attach the lid to the bottom of the enclosure.

Step 10 - Mount the Adapter

TouchDRO adapter communicates with the tablet wireless. This offers a lot of flexibility for adapter and tablet mounting. The following the tips below will ensure best performance and reliability: