The Plover HID protocol

Background

Currently most hobbyist steno machines use either the TXBolt protocol or the GeminiPR protocol, which are serial protocols running over USB CDC. The main reason why many end users want to use the steno protocols instead of just having their machines act like keyboards is that they don't want to enable/disable plover all the time to be able to use their keyboards.

However there are certain issues with those protocols:

1. There is no serial profile for BLE, so any bluetooth low energy device would need a custom protocol (GATT Service).
2. GeminiPR and TXBolt only send complete chords to the host, this makes us lose information that could be used on the plover side to easily implement features such as first-up chord send or auto-repeat of chords.
3. TXBolt and GeminiPR define a very restricted amount of keys. This isn't an issue for English Stenotype, but makes it so that something like a velotype machine won't be able to use those protocols.
4. Plover handles serial disconnects pretty badly, causing the user to have to reconfigure their serial port in settings. This is not a big issue but has led to some user confusion in the plover discord server.
5. There has been talk about creating hobbyist machines supporting variable key-pressure/lever-pressure. Many professional machines have configurable per-key pressure settings, but we could also design a protocol to send the key pressure and handling the configuration on the plover side.

Using HID

Arguably stenography input is Human Input, so making a steno machine a human input device would make sense. The pros of using HID are that it's supported both over USB, and also over BLE. By sending each state change instead of whole chords we allow for having features like first up chord send and auto repeat in plover. In the best of worlds we would have been able to just choose a good HID page for steno, but unfortunately no such standard page exists.

So instead we choose a vendor-defined page, and choose a usage code that no one else seems to use. It would of course be nice if this could be standardized in the future, but that is probably out of reach for the amateur steno community.

It would also have been nice to just define the usage page and usage, and allow the device itself to decide how its reports should be sent (how many buttons it has, if it has pressure information, and so on), and then parse the hid descriptor on the plover side. Unfortunately there isn't a good platform independent way to retreive the HID descriptors from a device.

Instead we choose an approach where the Usage Page and Usage uniquely define a report format that we call the "simple report format" consisting of a bitmap of 64 bits each encoding if a certain key/lever is pressed or not.

The HID Descriptor

{
     0x06, 0x50, 0xff,              // UsagePage (65360)
     0x0a, 0x56, 0x4c,              // Usage (19542)
     0xa1, 0x02,                    // Collection (Logical)
     0x85, 0x50,                    //     ReportID (80)
     0x25, 0x01,                    //     LogicalMaximum (1)
     0x75, 0x01,                    //     ReportSize (1)
     0x95, 0x40,                    //     ReportCount (64)
     0x05, 0x0a,                    //     UsagePage (ordinal)
     0x19, 0x00,                    //     UsageMinimum (Ordinal(0))
     0x29, 0x3f,                    //     UsageMaximum (Ordinal(63))
     0x81, 0x02,                    //     Input (Variable)
     0xc0,                          // EndCollection
}

Where the first byte of the Usage Page is 0xff (vendor-defined) and the second byte is "S" and the usage bytes are "TN", together encoding the string "STN".

Due to the fact that we can't get the report descriptors in a platform-independent way we fix the ReportID to 80 (0x50) so that we can distinguish the reports of devices sending multiple reports on the same endpoint.

The descriptor was generated by the HID Report Descriptor Compiler from the following python code:

from hrdc.usage import *
from hrdc.descriptor import *

usage = Usage('vendor-defined', 0xff504c56)
stenomachine = Collection(Collection.Logical, usage,
    Report(0x50, *(Value(Value.Input, ordinal.Ordinal(x), 1, logicalMin=0, logicalMax=1) for x in range(64))))

if __name__ == '__main__':
    compile_main(stenomachine)

A previous version of the protocol used the Button usage instead of the Ordinal usage. But linux HID stack also parsed those as mouse-button pressed over BLE, while it didn't do so over USB. Even though this is probably a linux bug it's easier for us to change the usage to one where that doesn't happen.

The keys

For historical reasons we reuse the names of the standard steno keys, but we add extra keys so that we reach 64 keys in total. The keys are ordered as follows:

   S- T- K- P- W- H- R- A- O- * -E -U -F -R -P -B -L -G -T -S -D -Z #

and are then followed by key X1-X26

So the first bit/byte of the report (after the report type byte) maps to key S- and the last bit/byte of the report maps to X26.

Sample Firmware

Sample firmware for the georgi is included in this repository. It defines a default keymap of the form:

X1 S1- T- P- H- *1 *3 -F -P -L -T -D
X2 S2- K- W- R- *2 *4 -R -B -G -S -Z
          #  A   O  E  U  #

Plover HID Inspector

@jthlim has created a WebHID based tool for inspecting output from any Plover HID machine. This tool can be used to help debug any Plover HID protocol issues if you're adding Plover HID to your own board. The tool can be found here and required a web browser supporting WebHID.

Future Work

• Implement this protocol for QMK.
• Implement this protocol for ZMK.
• Create a Usage Page/Usage for lever machines sending variable lever pressure.
• Look into using Output Reports for sending things to the steno machine from plover.