The Crow Protocol

The Crow protocol is designed to allow a PC to control several microcontrollers using a single serial line. The protocol itself has a simple half-duplex, command/response format with basic error checking using Fletcher-16 checksums.

In Crow, the top-level software that generates commands is called a client. The client runs on the host, and it passes the command to a host implementation. The host implementation creates a command packet and sends it over the serial line. A device implementation receives the packet and then passes the command to a service running on that device. The service performs the command and may send a response, depending on the circumstances.

A command has the following properties:

• address: 1-31 for a specific device, or 0 to broadcast to all devices.
• port: 0-255, where open ports are registered to specific services.
• response expected: if true, the host will wait for a response; if false, the device must not respond.
• payload: 0-2047 bytes of binary data.

A response has just one property, the payload, which is also 0-2047 bytes of binary data.

Links

The specification can be found at https://github.com/chris-siedell/CrowStandard. As of April 2018 version 2 is in development. The specification for the CrowAdmin service can also be found at this page. The CrowAdmin service defines ping, echo, hostPresence, getDeviceInfo, getOpenPorts, and getPortInfo commands.

A Python 3 host implementation can be installed using "pip install crow-serial". The source page is at https://github.com/chris-siedell/PyCrow and the Python Package Index page is at https://pypi.org/project/crow-serial/.

A minimalist device implementation for the Parallax Propeller P8X32A can be found at https://github.com/chris-siedell/PropCR. PropCR allows only one open service port per instance (in addition to the CrowAdmin service using port 0). Another caveat: PropCR uses non-standard byte ordering for its command payloads. The PyCrow (aka crow-serial) host has a propcr_order option that will take care of the necessary re-ordering.