Polyphonic External Scale protocol, or PES, is a protocol developed Aria Salvatrice that allows scale information to be carried on a single cable between different VCV modules that support this protocol.
Here is Aria's original post
Here is a simple example where the Squiktronix Harmony II is setting the scale for Grande Quant:
One reason that PES is possible is that is is limited to 12 note scales that repeat each octave.
PES is a VCV connection carrying 12 channels. The twelve channels correspond to the twelve notes, starting with C on channel 0 and going up to B on channel 11.
For each channel the voltage will be 0 volts for a note that is not in the scale, 8 volts for a note that is in the scale, and 10 volts for the root of the scale.
Here is a screen capture of a C# Locrian scale, Where Grande Scale is setting the scale, Harmony II is following, and Grande Quant is following Harmony II:
As of this writing, the VCV 2.x Modules that I know of are:
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Squinktronix Harmony II. KSO Output is fully compliant. KSI input is compliant, but if the signal on the input does not correspond to a scale that Harmony II knows about, it will show an error, and ignore the input.
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Squinktronix Harmony. Now has a PES input.
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Grande Quant. The Scale input accepts a PES signal, and will display it. Since a quantizer doesn't know or care what the "root" is, the root (10 volts) is treated like any other note in the scale.
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Grande Scale. The output is a PES signal. Again, since Scale doesn't know or care about root, it is not sent. In this case everything is sent at 10V.
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PurrSoftware Meander, ModeScaleProgression, and ModeScaleQuant. All provide PES output.