GEX's low-level communication protocol

Framing layer

GEX uses TinyFrame to form and parse data frames.

Frames are normally sent and received using two USB bulk endpoints (VCOM or raw access) or through UART.
Transmitted frame length is virtually unlimited
Received frame length is limited by the Rx buffer size. This is overcome using a Bulk Write function.

Frame structure

See the TinyFrame documentation for detailed description of the frame structure and protocol functions.

Here's the configuration used by GEX:

,------+----------+---------+------+------------+- - - - -+------------,
| SOF  | frame_id | pld_len | type | head_cksum | payload | pld_cksum  |
| 1    | 2        | 2       | 1    | 1          | ...     | 1          | <- size (bytes)
'------+----------+---------+------+------------+- - - - -+------------'

SOF byte is 0x01
Checksum = inverted XOR of all bytes

USB bulk transfers have CRC error checking built-in. The checksum field is mainly used to catch implementation mistakes, such as a CR-LF transformation.

Frame ID is incremented with each transaction (message, request/response, or longer). The highest ID bit identifies the peer who started the transaction ("master", "slave").

Type defines the meaning of the frame within a higher level protocol. A list of all types is attached below.

Message types

General messages

0x00 ... SUCCESS - Generic success response; used by default in all responses;
- Payload:
  - optional, differs based on the request
0x01 ... PING - Ping request, used to test connection
- Response:
  - SUCCESS frame with an ASCII string containing the GEX version and the hardware platform name
0x02 ... ERROR - Generic failure response, used when a request fails to execute
- Payload:
  - ASCII string with the error message

Bulk transfer (large multi-frame transfer)

Bulk transfer is used for reading / writing large files that exceed the TinyFrame buffer sizes.

0x03 ... BULK_READ_OFFER - Offer of data to read.
- Payload:
  - u32 - total len
0x04 ... BULK_READ_POLL - Request to read a previously announced chunk (BULK_READ_OFFER)
- Payload:
  - u32 - max chunk size to read
- Response: a BULK_DATA frame (see below)
0x05 ... BULK_WRITE_OFFER - Offer to receive data in a write transaction.
- This is a report of readiness after some other frame introduced a request to write bulk data (e.g. writing a config file)
- Payload:
  - u32 - max total size
  - u32 - max chunk size
0x06 ... BULK_DATA - Writing a chunk, or sending a chunk to master.
- Payload:
  - a block of data
0x07 ... BULK_END - Bulk transfer is done, no more data to read or write. Recipient shall check total length and discard the received data on mismatch.
0x08 ... BULK_ABORT - Abort and discard the ongoing transfer (sent by either peer)

Unit messages

Units are instances of a particular unit driver. A unit driver is e.g. Digital Output or SPI. Each unit has a TYPE (which driver to use), NAME (for user convenience) and CALLSIGN (for messages)

In the INI file, all three are shown in the unit section: [type:name@callsign]

Units can accept commands and generate events. Both are identified by 1-byte codes.

0x10 ... UNIT_REQUEST - Command addressed to a particular unit
- Payload:
  - u8 callsign
  - u8 command
  - rest - data for a unit driver
0x11 ... UNIT_REPORT - Spontaneous report from a unit
- Payload:
  - u8 callsign
  - u8 report type
  - u64 usec timestamp
  - rest - data from unit driver

System messages

Settings management etc.

0x20 ... LIST_UNITS - Get all active unit call-signs, types and names
- Response:
  - u8 number of units
  - repeat for each unit:
    - u8 callsign
    - cstring type
    - cstring name
0x21 ... INI_READ - Read the ini file via bulk
- starts a bulk read transfer
- Response:
  - a BULK_READ_OFFER frame (see above)
0x22 ... INI_WRITE - Write the ini file via bulk
- starts a bulk write transfer
- Response:
  - a BULK_WRITE_OFFER frame (see above)
0x23 ... PERSIST_CFG - Write current settings to Flash
- This is the equivalent of replacing the lock jumper / pushing the button when the virtual filesystem is enabled.
- If the virtual filesystem is enabled, this also closes it.

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