Machine Safety

Lizard implements the following features to increase machine safety.

Checksums

Each line sent via the command-line interface can and should be followed by a checksum. Lizard will omit any lines with incorrect checksums. Any output is as well sent with a checksum.

The 8-bit checksum is computed as the bitwise XOR of all characters excluding the newline character and written as a two-digit hex number (with leading zeros) separated with an @ character, for example:

Line	Bitwise XOR	Result
1 + 2	0x31 ^ 0x20 ^ 0x2b ^ 0x20 ^ 0x32 = 0x28	1 + 2@28

Keep-alive signal

The core module provides a property last_message_age, which holds the time in milliseconds since the last input message was received from UART0, parsed and successfully interpreted. It allows formulating rules that stop critical hardware modules when the connection to the host system is lost.

The following example stops a motor when there is no serial communication for 500 ms:

when core.last_message_age > 500 then motor.stop(); end

Expander watchdog

The expander module provides a watchdog feature that restarts the port expander when it gets stuck and does not send messages anymore. After ping_interval seconds of no messages from the port expander, the expander module will instruct the expander to send a "__PONG__" message. If the expander does not answer within ping_timeout seconds, it will be restarted.

The reception of the "__PONG__" message is handled internally by the expander module and is not printed to the serial output. A similar technique can be used by the main computing unit to check if the core microcontroller is still responsive and restart it otherwise.

If a proxy module is active, the ping_interval will never elapse, because the expander module receives messages via the proxy message handling. Thus it will not need to ping the expander explicitly.