TouchDRO TDA-400/420 Adapter Manual

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

TouchDRO TDA-400 and TDA-420 are the latest generation of our scale interface adapters designed for use with the majority of modern optical and magnetic DRO scales. Thanks to the powerful 32-bit dual core processor and state-of-the art software, these adapters offer unparalleled performance and capabilities that can't be found in any other DRO unit on the market.

TouchDRO TDA-400/420 Adapter Front


TouchDRO TDA-400 is a four-axis DRO scale adapter, designed to work with most modern optical and magnetic DRO scales.


TouchDRO TDA-420 is an enhanced version of the TDA-400 adapter. It includes all of the features listed above, and adds two additional flexible TTL inputs that can be configured for a few different use cases.

TouchDRO TDA-420 Adapter (top) has two additional inputs on the back.

What is Included

When you receive your TDA scale adapter, it will be ready to go:


Four RS-422/TTL Scale Inputs

Both adapters come 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. Many DRO scales also include a reference line, often labeled R or Z, that outputs a pulse at a predefined distance (more on this later).

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 a good to have feature but not strictly necessary, unless the scale cables are very long (multiple tens of meters).

Support for Reference Track

The four primary scale inputs include support for a reference track. Good quality DRO scales usually come with a reference track with reference marks spaced at predefined distance, or a single "home" mark, usually in the middle of the scale. Since reference marks are usually very precise, they can be used in TouchDRO to restore the machine's coordinate system to a known position.

The reference input is turned off by default. When it's turned on from the TouchDRO application, the adapter will listen for a reference pulse for up to 30 seconds. When the pulse is detected, the axis readout is zeroed out and the reference input is turned off.

Auxiliary TTL Inputs

TouchDRO TDA-420 adapter comes with two additional flexible TTL lines that can be used for a combination of the following:

The home switches work similarly to the reference track, but are always active. When the line is pulled to the ground, the axis is zeroed out immediately.

Position Retention on Power Loss

Both TDA adapters are equipped with non-volatile memory (EEPROM) and a power loss detection circuit that records the last known scales position when the adapter is powered down. When the power is restored, the adapter will check the data integrity, verify that the scales were not in motion, and restore the positions. If the checks fail, it will send an error message to the application alerting the machinist of the potential position loss.

BlueTooth and USB Connectivity

TouchDRO TDA adapters come with a BlueTooth 4.2 transceiver and a high speed USB bridge. The firmware can use either BlueTooth or USB to send the data, or both connection modes can be used simultaneously. For example, BlueTooth can be used to connect to a tablet and USB can be plugged into a computer for real-time data capture.

Support of Over-the-Air Updates

The software running on the TouchDRO adapters (also called firmware) will continue to be updated and receive additional functionality. This firmware can now be updated directly from the TouchDRO application. When a new firmware is released, the app will ask if you want to update the firmware. If the update is approved, the code will be uploaded to the microcontroller.

Access to TouchDRO Plus Application

TouchDRO Plus adds a number of advanced features that are not available with the free version of the application. The TDA adapters include the activation license for the Plus features. When you first connect to the adapter, TouchDRO application will detect the activation license, verify it against the activation server and turn on the features.

You will have an option to log into TouchDRO service with your Android credentials and tie the activation code to your account. This will unlock TouchDRO Plus for any other TouchDRO adapters used with the tablet that is logged into the account (some restrictions apply).

Main Input Pinout Options

Four main scale inputs on TouchDRO TDA-400 and TDA-420 use female DB-9 (D-Sub 9) connectors preconfigured for one of the scale peanuts listed in the table below.

Brand 1 2 3 4 5 6 7 8 9
Acu-Rite SENC Scales   A A' B B' OV 5V R R'
Electronica/EMS, PM MagExact +Z Z' 5V Shield Gnd A A' B' B
Ditron, Sino, Aikron, Easson A' 0V B' Shield R'/Z' A +5V B R/Z
Vevor A' 0V B'   Z' A 5V B Z
Generic Chinese Scales   0V       A 5V B R1/Z

For more detailed information plese refer to Supported Optical and Magnetic DRO Scales

Auxiliary Inputs

By default, Aux. 1 input is configured for two touch probe inputs, and Aux. 2 is configured for tachometer input. The table below shows all possible input configurations.

  Pin 1 Pin 2 Pin 3 Pin 4 Note
Touch Probe Gnd. 5V "p" "q" Aux. 1 only
Tachometer Gnd. 5V "t" --- Aux. 2 only
Quadrature Gnd. 5V A B Aux. 1 - "u", Aux. 2 - "v"
Limit switch X/Y Gnd. 5V X Y
Limit switch Z/W Gnd. 5V Z W
Serial data Gnd. 5V Clock Data

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.

This means that when an axis is moving, the worst-case position display latency is about 52 milliseconds. This includes the 40 millisecond delay from the refresh rate, 11 millisecond screen redraw time, and under 1 millisecond BlueTooth or USB data transfer time. The average latency will be about 25 milliseconds. For comparison, good quality digital calipers have a refresh rate that is 2-8 times slower.

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.

The two auxiliary inputs (TDA-420 only) can reliably handle combined quadrature signal frequency of 100 KHz. In other words, if both axes are moving, their combined signal frequency should not exceed 100KHz; when a single axis is moving, it can output a full 100KHz. This is equivalent to a 1-micron scale moving at 0.4 meters per second, which is on-par with most modern DRO units.

Reference, Touch Probe, and Home Switch Latency

When the reference lines, touch probe, and home switch line signal changes form on to off stage, the adapter must respond to the event and perform the appropriate action. This takes some amount of time. The TDA adapters can handle the interrupt in about 2 microseconds or less. This translates to some amount of repeatability error. For example, a home switch is triggered when a 1-micron scale is moving at 1 meter/second, the error will be 8 encoder steps, or 8 microns. At a more reasonable speed, the error will be well under one encoder step.

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. The measurement error is under 1%.

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 ---
Reference Trigger Latency 2 µs 2 µs 2 µs
Signal High Level 5 V 2 V 5.5 V
Signal Low Level 0 V -0.3 V 0.8 V
Auxiliary Inputs
Quadrature Signal Frequency 50 KHz --- 100 KHz
Min. Edge Separation 5 µs 5 µs ---
Probe/Home Switch Latency 2 µs 2 µs ---
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