Generic 48-Bit Chinese Scales

Generic Chinese 48-bit linear scales were the mainstay of the DIY DRO scene in the early 2000s. As more affordable and faster iGaging DigiMag became available, these scales lost their market share and are far less common now. Nevertheless, they still pop up from time to time on some outlets.

These scales have a number of shortcomings that makes them less than ideal for a do-it-yourself digital readout. First and foremost, they use 1.5V power supply and signal level requiring additional circuitry for connecting to a 3V microcontroller. Second, they have a slow default refresh rate of 3Hz; although, some scales can be switched to a "fast" mode. Third, the frames are connected to the positive side of the battery, making grounding or mixing and matching the scales with modern 3V scales tricky. Finally, these scales are prone to a random data stream glitches and position jumps even under ideal circumstances.

Scale Models

The 48-bit Chinese scales come in a plethora of sizes and shapes and are sold under various brands. Unfortunately, they look very similar to the modern scales sold by Shahe, iGaging and alike. The easiest way to tell these scales apart from the modern models is to look at the battery and the data port. Newer scales use a 3 Volt cell battery that has the negative side connected to the frame and the data port that uses either USB Mini-B or USB Micro-B connector. In contrast, the older 48-bit scales use the same proprietary data port that can be found on most inexpensive digital calipers and a 1.5V battery with the positive side connected to the frame.

Technical Details

* Please note, the resolution of the data stream is 20,480 positions per inch, which is around 1.2 microns. In practice, these scales can't come anywhere near that resolution and are much closer to 10 microns or so.

Data Format and Protocol

These scales use a 48-bit data format first introduced by Sylvac. The data stream consists of two 24-bit words. The first represents the position in relation to the "absolute" origin, i.e. the arbitrary point where the scale was first powered up. The second word represents the incremental position in relation to the "Zero" point. The position is represented as a signed binary number with the least significant bit first with a theoretical resolution of 20,480 positions per inch.

While there are minor variations between different manufacturers, as a general rule, these scales have a default refresh rate of about 3 Hz. Some specimens have a fast mode that can be triggered using a hardware "Mode" button (if present) or by momentarily pulling up the data line. Unfortunately, there is no way to tell if a particular scale supports the fast mode, and even if it does, there is no guarantee that the readout will be stable enough.

Connection Scheme

The 48-bit Chinese scales use the proprietary 4-pin data connector that is common on most inexpensive Chinese calipers (shown below). While this connector is, no doubt, very cost-effective for the manufacturers since t utilizes the traces on the PCB and an opening in the plastic housing as the female connector. Unfortunately, compatible data cables are relatively rare and expensive as well as notoriously unreliable due to the poor quality of molding and flex.

When connecting these scales to a DRO adapter, it's important to remember a few key points:

First, as a general rule, the scales prefer to be connected to a high-impedance input without any pull-up or pull-down resistors, but this might wary by the manufacturer. In some cases, the stability might be improved by pulling the data and clock line up to 1.5V or down to the ground using 49K or larger resistor.

Second, since the scales use a 1.5V signal level, they are extremely sensitive to electrical noise. This necessitates proper shielding and grounding of the cables and the scales. Unfortunately, the frames of these scales are connected to the positive side of the battery, so one should be careful when planning the ground connections, or permanent damage to the scales might occur.

TouchDRO Compatibility

TouchDRO Mixed Scale Firmware supports the protocol and can be used with these scales. Since the scales use 1.5V power source and output 1.5V signal, a voltage level shifter circuit is required. Additionally, TouchDRO firmware does not automatically trigger the fast mode.

Pros

• Higher theoretical resolution
• Stainless steel frame and encoder housing
• Available in a vertical and horizontal configuration

Cons

• Relatively high cost
• Low default refresh rate
• Limited lengths available
• 1.5V signal level
• Prone to random position jumps

Final Words

The 48-bit scales used to be the mainstay of the do-it-yourself DRO scene. Their main appeal was the relatively low cost when compared to the contemporary quadrature scales. On the other hand, these scales have a few major shortcomings. First and foremost, the 1.5V single level is very susceptible to noise and interference. Combined with their high-frequency data clock rate and the inexplicable random position jumps, these scales make a poor choice for a DIY digital readout, especially given the lower price and better reliability of the modern iGaging and Shahe scales.