21 Bit Protocol Scales

21 Bit Protocol

Probing the remote LCD readout's USB connector with a voltmeter indicates that pin 1 is connected to the positive side of the battery (3V) and that pin 4 is connected to the negative side of the battery (ground).  A capture with an oscilloscope with the scale in operation connected to the remote LCD readout reveals 3V single-ended digital signals are present on pins 2 (red trace) and 3 (yellow trace).  The signals occur in repetitive bursts that repeat approximately every 7 milliseconds or at a frequency of 143 Hz.

Zooming in on one of the bursts shows the signal in more detail.  There are 21 clock pulses on the red trace (pin 2) and a digital signal on the yellow trace (pin 3).  By using our previous knowledge of Chinese scales, we can guess that the signal is a synchronous serial signal that gives the scale's position in binary with the least significant bit (LSB) first.  This guess turns out to be correct.  The Chinese scales send two 24 bit binary values with one being the absolute position and the other being the relative position each expressed as 20480 counts per inch (CPI).  Given that we have a single 21 bit value, we can also guess that the single 21 bit value sent by the scale is an absolute position with 3 bits less precision to yield 2560 CPI (20480 / 23).  Again, this guess turns out to be correct.

The last thing to determine is the source and speed of the clock signal.  The oscilloscope trace shown at left gives us a clock speed of about 9 kHz which is much slower than a Chinese scale at around 90 kHz.  In most synchronous serial protocols, the entity that provides the clock is designated the master and the one that receives the clock is the slave.  For Chinese protocols, the scale provides the clock so it is the master.  By disconnecting the iGaging scale from the remote LCD readout, we find that the remote LCD readout is providing the clock so the scale is a slave.  This difference means that the DRO/DPU-550 with OpenDRO must provide an appropriate clock to the scale if it takes the place of the remote LCD readout.