21 Bit Protocol Scales


A company named iGaging has introduced a new series of digital scales that rivals the affordability of Chinese import scales from Harbor Freight, Shars, and other sources.  They come in lengths from 6 inches up to 35 inches.  ShumaTech performed a technical assessment of these scales and is pleased to report that we were able to adapt the DRO-550 and DPU-550 running OpenDRO for use with these scales.  A picture of a iGaging model 35-706 6" digital scale is shown at left along with its included remote LCD readout.  If you are interested in purchasing iGaging scales, you can find them at Grizzly Industrial, Amazon, Eagle America, and several other on-line stores.  ShumaTech is not affliated with any of these stores.

The internals of the iGaging scales reveal that this scale is based on the same capacitive design as the Chinese import scales.  The iGaging scales are in fact made in China too.  The capacitive design was originally developed by Sylvac in the early 1980's.  The scale internals pictured at left reveal the same 200 mil placement of coupling electrodes on the scale beam along with the eight signal electrodes repeating every 200 mils.  The large receiver electrode that is capacitively coupled to the signal electrodes via the coupling electrodes in the scale beam is found on top.  If the reader is interested in more technical details on the capacitive scales, lots of information can be found in patent 4420754 and other patents  that reference it.

One other interesting attribute of these scales is that like all other Chinese scales, the beam is conductive and connected to the battery.  In the case of the iGaging scales, the beam is connected to the negative side of the battery.  This means that if you mix iGaging scales with 24 bit or BCD7 scales that have their scale beams connected to the positive side of the battery, then you must galvanically isolate one scale type or the other.  If you don't, then the power supply on the DRO will be shorted and will shut down until the short is removed.

The connector on the circuit board is a USB mini B female but beware that the protocol is NOT USB as we will show on the next page.  Plugging these scales into a USB port on a PC could damage them.  The cable that connects the scale to the remote LCD readout is a standard USB mini B male to USB mini B male.  The remote LCD readout provides several functions such as metric/inch conversion, zeroing, and fractions.  It also supplies 3V power from two coin cell batteries to the scale.  Note that the readout is not necessary to read the position from the scales since all capacitive processing and digital interface circuitry is on the scale PCB.