Choosing the Right TouchDRO Adapter for Your Scales

Tuesday, September 27, 2016

I started offering pre-made controllers/DIY kit earlier this year, and have received a number of emails asking to clarify which DRO controller to chose and why. At the time of this writing I offer three versions of the controller hardware: two as a pre-assembler board and one as a DIY kit. Each controller is designed to interface with specific set of scales and uses firmware that is specifically optimized for that particular hardware. The purpose of this post is to explain what each controller was designed for and what are the benefits/tradeoffs of using it as opposed to the other controllers.

At the time of this writing TouchDRO can interface with iGaging DigiMag/AccuRemote/Shahe "Remote DRO" scales, Chinese calipers, Sylvac-type scales (AKA "Chinese Scales"), iGaging Absolute DRO Plus scales and various glass and magnetic scales that provide RS-422 (TIA/EIA-422) or TTL quadrature signal. The set of pre-made boards and the kit cover the same range of scales as the DIY (scratch-built) controllers, but there is no clear 1:1 mapping between the two. The do-it-yourself controllers are designed to be easy to build from commonly available components, which required some compromises. Pre-made boards use different firmware that is optimized for the characteristics of the specific firmware and the primary design goal was to provide best performance and long-term reliability from the controllers.

Please note that the firmware available from the downloads section is provided free of change for personal use. If you intend to use the controllers in a commercial setting or re-sell them, the free version can't be used.

Pre-Made vs. DIY

Decision between building a controller from scratch vs. buying a board or a kit comes down to the budget. Across the board the pre-made boards will perform better (more reliably) in the shop environment. The PCBs are laid out to deal with electromagnetic interference by using ground planes on both sides, short SMT leads, matching impedance traces, bypass capacitors, and regulated power supply in close proximity to the chips. Moreover, the boards will come to your door programmed and tested. In contrast, a DIY build on a breadboard will be more noise prone by the virtue of long component leads and wires and it will be up to you to build, program, test, and potentially troubleshoot the board.

The downside of the pre-made boards is the cost: the PCBs are fabricated and assembled in the USA to strict IPC quality standards using name-brand components; each board is then tested individually (connected to a tablet and a set of scales and ran through its paces). All these costs are add up, so the finished board will cost 2-3 times more than a scratch built one.

Midex Scale Kit

Finished Mixed Scale DRO Controller

The Do-it-yourself DRO Mixed Scale Controller Kit is designed to work with a mix of capacitive scales, such as inexpensive calipers, "Chinese" DRO scales, iGaging DigiMag/Absolute DRO+, etc. that use either 1.5V or 3.3V supply voltage. It contains the voltage shifter circuit, MCP430 MCU with some supporting components and a voltage regulator/power supply running a bit more optimized firmware. The main benefit of the kit is the convenience of a pre-made PCB and the pre-programmed chip with optimized firmware. Moreover, noise characteristics of the kit are better than what you'd get with a breadboard build due to the copper fill.

I intentionally designed this as a kit to keep the cost low, so the current sale price is very close to what you'd spend buying the components individually. This is especially true if you try to source good name-brand components, not counting the PCB.

There are a few important points about the kit:

If you decide to mix iGaging scales with calipers or other 1.5V scales, make sure that the frames are isolated from the machine and each other. The frame of the iGaging scales is connected to the ground, whereas the frame of 1.5V scales is connected to V+. If they touch you will burn out the controller and/or one or both scales.

Side Note: If you intend to use the kit with a set iGaging scales, the dedicated iGaging controller [described below] will work much better. iGaging/Shahe scales are very sensitive to noise and the purpose-built controller will perform many times better in that regard.

iGaging Controller

TouchDRO Adapter Board for iGaging Scales

TouchDRO Adapter Board for iGaging Scales is designed to work with the "AbsoluteDRO+" and "DigiMag" scales sold in the USA by iGaging. It can read up to four scales, directional tachometer and has support for touch probe input. The board is designed to be almost plug-and-play. I.e. if doesn't require any soldering, since the scales can be connected via the screw terminals.

The circuit is based on the MSP430 LaunchPad build for iGaging scales but has superior noise resistance. The PCB is fabricated with heavy copper fill on both sides, bypass capacitors and clean power supply regulator close to the chip to mitigate adverse effects of EMI and ground loops. Based on the feedback I've been receiving from the customers the controller doesn't suffer from the jumping and resetting issues that plagued the scratch-build version.

Side Note:iGaging DigiMag scales appear to be made by Chinese company called "Shahe" and are sold all over the world under that name. Additionally, scales marketed as AccuRemote are electronically identical to the generic Shahe scales but use better stainless steel frame, as opposed to the aluminum and plastic construction. When rigidly mounted to a mill or a lathe there is no difference in accuracy or durability between DigiMag and AccuRemote scales.

TouchDRO Quadrature Adapter

TouchDRO Quadrature Adapter

The TouchDRO Quadrature Adapter is designed to work with glass and magnetic scales that output 5V quadrature signal. Vast majority of the modern scales on the market fall into this category. This includes most Ditron, Easson, Sino and alike, as well as many scales from other manufacturers.

The board can be purchased as a "No-Soldering" kit or by itself. In the former case it will come with four D-Sub 9 harnesses that use press-fit pins and require no soldering.

The main benefit this board provides over the DIY approach is the use of dedicated differential signal decoders. This is important for two reasons. First of all, many of the scales have very strict impedance and back EMF requirements. 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.

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.

Side Note: While the board works equally well with differential and single-sided input, differential input is preferable from the noise immunity standpoint. I won't go into technical details, but the bottom line is that the complementary channels cancel out noise on the line, which is necessary for very long runs of wire. For standard 6-12 foot cable lengths single-sided input will likely be adequate, but given the option I would choose scales that provide differential output.

Summary

If you enjoy building things from scratch, can solder, feel comfortable uploading firmware to a microcontroller, and have the tools (and patience) to troubleshoot noise issues, scratch building a controller is an obvious choice. To decide which version to build, please refer to the Do-it-Yourself DRO Scale Controllers Overview

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 scales I would strongly recommend getting the pre-made TouchDRO iGaging Adapter controller. 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 largely immune to those issues and handles the noise very well.

For other capacitive scales you will need a voltage shifter circuit. If you already have the parts, building one the a breadboard isn't too difficult, as shown in the build instructions. Otherwise you might be better off getting the Mixed Scale DRO Controller Kit. By the time you buy the parts the difference in the price will pretty small (if not negligible) but the kit will be easier to build and will likely perform better.

For quadrature scales the choice of the controller is a bit less clear cut: the kit (or the DIY voltage shifter circuit) will work good enough for most modern 5 micron glass scales. In fact the DRO on my mill uses the voltage shifter circuit and I've been using it for a couple of years without any problems with a set of Easson scales. 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.

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