Choosing the Right TouchDRO Adapter for Your Scales

Tuesday, September 27, 2016

One of the main design goals for the TouchDRO project was to create a flexible modular do-it-yourself digital readout for hobbyists and makers alike. Since raw scale position decoding happens in the adapter, one of the first decisions when setting up a TouchDRO system is the selection of the adapter. At the time of this writing there are two versions of pre-made adapter board and four different do-it-yourself options. All those choices can be confusing if you're new to the TouchDRO ecosystem, so let's look at the benefits and tradeoffs of each of the six options.

UPDATE: February 24, 2019 - the original content of this post has been updated to reflect the currently available adapter board choices and firmware versions.

Licensing and Legal Stuff

Before digging deeper into the adapter options, let's cover some legal stuff. TouchDRO is a free application and I provide several free versions of the adapter firmware. The app is distributed via Google Play Store and is free for personal or commercial use, subject to Google Play Terms of Service. Free versions of the firmware are free for personal use only. In other words, for personal use you can choose between the pre-made boards or a DIY approach using one of the free firmware versions; for commercial use you can't use free firmware and need to buy a premade board.

Building vs. Buying a TouchDRO Adapter

The main factors in deciding between getting a pre-made board or scratch-building a DRO adapter will likely come down to your budget and comfort level with soldering, firmware flashing and troubleshooting. Scratch-building a DRO adapter can be less expensive, costing between half and one third of the pre-assembled adapter price. A ready-to-go adaper board is less hassle, on the other hand.

Pre-Made TouchDRO Adapter

Pre-assembled TouchDRO adapters are a better choice for people who need good reliability and people who aren't proficient with electronics. Even for those who have good soldering skills and equipment the pre-made boards have a few important benefits:

  • Pre-made boards use purpose-designed printed circuit boards that are laid out to minimize noise and electromagnetic interference, so they perform much better in the shop environment. This is especially true for the capacitive scales. A lot of people who built do-it-yourself have issues with random scale resets and position jumps due to noise and interference. These problems are virtually non-existent with the premade boards.
  • The firmware used by the pre-made boards has a number of tweaks and optimizations that are not included in the free version. For instance, the firmware that ships with the iGaging and Shahe DRO Adapter has more advanced tachometer implementation, adaptive scale refresh rate and supports mixing and matching of five different types of 3V capacitive DRO scales.
  • The board are professionally assembled using quality printed circuit boards, name-brand components, including Panasonic electrolytic capacitors, Texas Instruments ICs, and are inspected to pass IPC-A-610 Class 3* workmanship standard. This ensures long service life, low failure rate and excellent reliability.
  • Before shipping an order I personally inspect every board and run it through a comprehensive testing routine. This means that when you receive the adapter board you ordered you won't have to waste your time troubleshooting soldering issues, part defects, etc.

* IPC-A-610 Class 3 is usually required for medical devices, aerospace equipment etc. Good quality industrial equipment generally uses Class 2, so Class 3 a big step up in this case. You can read more about Class 2 vs. Class 3 here.

Scratch Built Do-It-Yourself DRO Adapter

As mentioned earlier, scratch-building a DRO adapter is significantly less expensive. On the low-end, a basic adapter for iGaging DigiMag and EZ-View DRO scales (i.e. scales that use the 21-bit protocol) can cost under $20 using MSP430 LaunchPad board and a HC-05 BlueTooth transceiver. Do-it-yourself adapters for glass/magnetic or 1.5V DRO scales will require significantly more parts and a prototyping board, and will likely cost around $50.

Another good candidate for a do-it-yourself DRO controller is a setup that uses 1.5V scales or a mix of 1.5V and 3V scales. There is no ready-made TouchDRO board that can interface with 1.5V scales, so scratch-building a buffer circuit. Please refer to Build Instructions for Voltage Shifter Circuit for more details.

Do-it-yourself DRO adapters can suffer from a few problems when used with capacitive scales. The reason being that capacitive scales are very sensitive to line noise and benefit greatly from good PCB layout with ground fill and properly located bypass capacitors, which is hard to do when using a breadboard. Glass scales are generally not affected by the PCB layout as much and work very well with DIY DRO setups. The only minor downside is that proper differential line drivers are not available in through-hole packages, so building an adapter for scales that use RS-422 (TIA/EIA-422) is much more tricky and somewhat more expensive.

Adapter for Glass and Magnetic Scales

Vast majority of the modern Glass and Magnetic DRO Scales use quadrature output, which is supported by the TouchDRO Adapter for Glass Scales board as well as the MSP430 LaunchPad Quadrature DIY DRO Adapter. The difference between the free firmware and the version that comes pre-installed in the adapter board are pretty minimal. The main advantage that the pre-made board offers is the use of a purpose-designed differential quadrature input driver IC. This is important for two reasons. First, many of the scales have very strict impedance and back EMF requirements. Pushing the scales too far out of their designed envelope can lead to shortened scale life or even damage. Second, the decoder chips can handle much faster signal rates (on the order of 32 MHz per channel).

Unfortunately there is no scratch-built equivalent of this board since the ICs used for the differential signal decoding aren't available in through-hole package. A DIY adapter using a comparator or a simple buffer is usually adequate but is far from ideal for those scales.

Tip: The best way to determine if the particular scales you have or are planning to purchase are compatible with TouchDRO is to refer to the scale's specifications sheet. Ideally there should be a drawing that shows the output signal that looks similar to one of the drawings below. Otherwise, if the scale is advertised to output RS-422, EIA-422-A or something similar, the scale outputs differential signal. Otherwise, if it mentions TTL, it's likely that the scale outputs single-ended quadrature signal. Another clue might be provided by the pin function diagram. You should see pins marked A, B, Z, R, 0V, +5V, Ground, Shield, etc. For differential scales you might also see A-, B-, A', B', etc. If that is the case, the scales will be compatible with the Quadrature controller. On the other hand if you find pins market "Clock", "Data" or similar, the scale is almost certainly incompatible.

Adapter for 3V Shahe and iGaging DRO Scales

There are three families of 3V "Remote DRO" scales sold by Shahe, iGaging and a few other manufacturers. Scales like the original iGaging DigiMag "Remote DRO", iGaging EZ-View DRO Plus, AccuRemote Digital Remote Readout and Shars "Digital Machine Aluminum/Stainless Steel Scale" use proprietary 21-bit data format. iGaging Absolute DRO Plus scales use a modified version of Mitutoyo protocols. Finally, the newer Shahe "Round Head" and "Square Head" Linear DRO scales use a format similar to the cheap Chinese calipers.

Scales that use the 21-bit protocol are fully supported by the MSP430 LaunchPad Basic DRO Adapter while the Adapter Board for Shahe and iGaging Capacitive DRO Scales supports all three scale families as of firmware version 2.0.

In addition to supporting more scale protocols, the Adapter Board for Shahe and iGaging Capacitive DRO Scales comes with optimized firmware that offers better stability and faster position refresh rate. More importantly, due to more optimal PCB layout and liberal sprinkling of bypass capacitors the board is a lot less prone to EMF and other shop noise.

Adapter For 1.5V Scales

The only way to interface 1.5V scales, such as the old-style Chinese linear scales with integrated display or digital calipers is to build a 1.5V to 3V buffer circuit similar to the one described in the Build Instructions for a DIY Voltage Shifter Circuit post and use the "Mixed Scale" firmware. Currently the firmware supports Sylvac 48-bit protocol, BIN6 protocol and 21-bit iGaging protocol.

Summary

If you are looking for a budget friendly DRO for personal use, enjoy building things from scratch, know how to solder and upload the firmware to a microcontroller, and have the tools (and patience) to troubleshoot noise issues, then scratch building a controller is an obvious choice. Please refer to the Do-it-Yourself DRO Scale Controllers Overview for more details.

On the other hand, if you're not comfortable with electronics, buying a pre-made board provide much less frustrating experience.

If you fall somewhere in the middle, here are my recommendations:

For iGaging and Shahe 3V scales I would strongly suggest getting the pre-made TouchDRO Adapter for Shahe and iGaging scales. If you browse the site you'll see many comments from people having problems with iGaging scales related to the noise that can be very frustrating to track down and fix. Pre-made iGaging controller is much more resilient and handles the noise very well. For other capacitive scales you will need a voltage shifter circuit.

For quadrature scales the choice of the controller is a bit less clear cut: the DIY voltage shifter circuit will work good enough for most modern 5 micron glass scales. I've used it on my milling machines for a couple of years without any ill effects and was pretty happy with the performance. On the other hand, there are scales on the market that have very particular input circuit requirements and can be easily damaged otherwise. Many of the magnetic scales, for instance, fall into this category. For such scales the pre-made TouchDRO Quadrature Adapter is a safer choice. Moreover, for setups using 1 micron scales I would opt for the pre-made controller as well, since it can handle faster signal rates better.