From Piano to Plexus

Piano keys carry multiple names as shown below.

  • 7 naturals
  • 7 sharps
  • 7 flats
  • 7 double-sharps
  • 7 double-flats

The Tonal Plexus keyboard layout is based on the traditional piano keyboard pattern of 7 white and 5 black keys, with standard fifths-based names as shown below.

All standard note names correspond to unique, non-dulpicate keys on the Tonal Plexus. Extended fifths-based note naming leads to a new set of enharmonics.

  • 6 triple-sharps
  • 6 triple-flats

These 12 new enharmonic keys are shown at the top and bottom edges of the layout, the blue keys in the images shown here.

  • 211 keys per octave, arranged in 12 staggered columns
  • 41 regions of 5 keys each = 205, plus 6 duplicate enharmonic keys


12 Column Zig-Zag

The piano keyboard pattern can be seen within the Tonal Plexus layout in a zig-zag. The Plexus pattern is unique, but similar patterns have been used before, most notably by the 19th Century microtonal organ builder R.H.M Bosanquet. An excellent advantage of this design is uniformity of interval patterns regardless of key.

Key Diameter and Elevation

Tonal Plexus keytops vary in diameter and elevation, arranged symmetrically in 41 regions per octave. The center key of each region has a concave surface and is the largest in diameter. The other keys are smaller and have flat tops. These variations of diameter and elevation provide a tactile terrain for navigation of the keyboard by touch. In the default Master Tuning, the variations correspond to JND inflections of a given letter name, allowing harmonic intervals to be easily found by touch alone.

MIDI Mapping

OCTAVE 1 2 3 4 5 6 7 8
CHANNEL A 13579111315
CHANNEL B 246810121416

MIDI Notes and Channels

Each octave of a Tonal Plexus keyboard uses two MIDI Channels, each assigned to six columns of keys. These two MIDI Channels are shown as CHANNEL A and CHANNEL B at right. The MIDI Note numbers are the same in each octave, and the MIDI Channels are fixed according to the size of the keyboard. The chart above shows MIDI Channel assignments for each size of Tonal Plexus keyboard. The keyboard responds to these MIDI Notes and Channels from external devices connected to MIDI IN.

Unused MIDI Notes

MIDI Notes 0-104 are used from Channel A and MIDI Notes 0-105 are used from Channel B. This leaves 22 MIDI Notes (105-127) unused on Channel A and 21 MIDI Notes (106-127) unused on Channel B. These 43 unused MIDI Notes correspond to 43 unused Tuning Table Registers for each Tonal Plexus octave. Although these free memory registers are ignored by the keys of a Tonal Plexus keyboard, they may be accessed from an external keyboard by sending the correct MIDI Notes on the correct MIDI Channels to MIDI IN.

U-PLEX Output / TPX Untuned Output Mode

Tonal Plexus keyboards normally send tuned output corresponding to MIDI Note and Pitch Bend data stored in a Tuning Table; however, when the top panel TUNED OUTPUT switch is set to OFF, the raw MIDI Note and Channel data shown here is sent to MIDI OUT. This data can then be received by an external processor such as a software sample player to result in retuned output. U-PLEX keyboards send only untuned data.

Default Master Tuning Duplicate Keys

In the default Master Tuning of 205ET, the following pairs of keys are tuned to the same pitch: Channel A 17 & 18, 68 & 69, 104 & Channel B 0. Channel B 35 & 36, 51 & 52, 87 & 88. These are the keys referred to as duplicate keys in TPXE software. These keys are not hard-wired as duplicates; custom user tunings can have any key mapped to any pitch.