A working draft the the design for Moppy 2.0

The general goals of Moppy 2.0 are:

• To modularize the code a bit (i.e. breaking core functionality into a separate library)
• Improve user experience (i.e. better GUI, easier setup)
• Support more musical devices (e.g. stepper motors, relays, HDDs) on more platforms (e.g. Raspberry Pi, Python)

To that end, the plan is to create slightly more robust message protocol that offloads a tiny bit more logic onto the device-side of things, to free up the controller library to be a little more generic. The GUI will be rewritten separate from the core logic code, and also allow remote configuration of the receiving devices. And generally the project is going to be restructured a little to allow contributions for other platforms and instruments.

Moppy messages have a 4 byte preamble, followed by a 1+ byte message body

Byte	Required?	Example	Contents
Start of Message	Yes	01001101	Always contains 01001101 and marks the start of a Moppy message
Device Address	Yes	00000001 (Device id 1) 00000000 (System message)	First half of device address. 00000000 indicates a system-wide message.
Sub-address	Yes	00000011 (Sub-address 3)	Second half of device address. Ignored if the previous byte is 00000000 indicating a system message. By convention, the first sub-address should be 00000001, and 00000000 should be reserved for device-wide commands.
Length	Yes	00000010 (Two byte body)	Indicates the number of bytes following this byte that make up the message body. For example, a command with two payload bytes would have a length of 3.
Command	Yes	00001010	The command for this message (see table below)
Payload	No	01101011 00100010 ...	Zero or more bytes containing relevant payload information for the given command.

These commands should be addressed to 00000000 (the sub-address should be ignored). Where possible commands are maped from relatively-equivalent MIDI commands (e.g. Start and Stop sequence) to ease conversion.

Command	Command Byte	Payload Bytes	Description
Ping	0x80	None	Requests a pong message from any Moppy devices on the network
Pong	0x81	[DeviceAddress] [MinSubAddress] [MaxSubAddress]	A pong message returned by devices when requested by the controller. The first payload byte indicates the device address, the second indicates the minimum subAddress (not including 0x00, which is a special case), and the third the max subAddress. Note: It is allowed for multiple devices to have the same address, possibly with a different range of sub-addresses
Reset	0xff	None	Instructs all devices to halt, and return to start / ready state
Sequence Start	0xfa	None	Informs all devices that a sequence has started (should precede note messages)
Sequence Stop	0xfc	None	Informs all devices that the sequence has stopped. Devices should halt, but not necessarily reset to their start / ready state

These commands should be addressed to both a device-address and a sub-address. A sub-address of 0x00 indicates that the command is meant as a device-level command (e.g. The reset command should probably reset the whole device instead of just one sub-address), but actual implementation may vary. Where applicable, commands are mapped from the first four bits of the equivalent MIDI command (e.g. MIDI's 1001nnnn Note On is mapped to 0x09, Play Note)

Command	Command Byte	Payload Bytes	Description
Reset	0x00	None	Instructs device to halt and reset itself to a start / ready state
Stop Note	0x08	[Note Number]	Stops playing the indicated note
Play Note	0x09	[Note Number] [Velocity]	Plays note indicated by number (which aligns to the MIDI note numbering standard). MIDI supports only notes 0-127, so any numbers above 128 are probably undefined (but could be used?). Velocity is likewise from 1-127 (0 velocity is remapped as a Stop Note event)
Bend Pitch	0x0e	[MSB][LSB]	Bends the pitch of note on this device interpreting the two payload bytes as a signed 16-bit integer. The actual bending procedure is a bit funky and left up to the implementer.