How to send periodic CAN messages with a script

tip

This is useful for sending a variable to the CAN, if the variable is not present in the CAN protocol.

The send command transmits a single message. To send messages periodically, wrap send in a script loop built from the branch command and a delay.

This example periodically broadcasts two CAN frames every 200 ms:

• CAN ID 0x700 — a scaled command and a mode:
  • bytes 0-1: /driver/cmd as int16 (a float in range -1 .. 1 scaled to the full INT16_MIN .. INT16_MAX range)
  • byte 2: /driver/mode as uint8
• CAN ID 0x701 — two GPIO values read live from the device:
  • bytes 0-1: /common/gpio0 as uint16
  • bytes 2-3: /common/gpio1 as uint16

The script

# --- one-time setup: create the variables ---
var cmd_scaled float
var raw int16          # scaled command (INT16_MIN .. INT16_MAX)

var msg700.3 uint8     # frame 0x700: int16 cmd + uint8 mode = 3 bytes
var msg701.2 uint16    # frame 0x701: gpio0 + gpio1 = 4 bytes


# --- periodic loop, detached to keep the shell usable ---
{
    # scale the float command (-1 .. 1) to the full int16 range
    set cmd_scaled /driver/cmd*32767
    set raw cmd_scaled

    # pack frame 0x700: bytes 0-1 = int16 cmd (LSB first), byte 2 = mode
    set msg700.0 raw
    set raw raw:256
    set msg700.1 raw
    set msg700.2 /driver/mode

    # pack frame 0x701: read the two GPIO inputs from the device
    set msg701.0 /common/gpio0
    set msg701.1 /common/gpio1

    # transmit both frames over the first CAN interface (interface 3)
    send -i3 -f1 -s0x700 msg700
    send -i3 -f1 -s0x701 msg701

    delay 200
    branch s
}

note

• Interface 3 is the first CAN bus. See the Interface IDs table for other interfaces.
• Flag -f1 selects the broadcast datagram so that -s0x700 is used directly as the CAN ID. See What CAN IDs are occupied? before choosing an ID.
• Change the period by editing the delay value (in milliseconds).

Persistent script

Use a "script manager" in the emGUI to store the script persistently on the device. The script will then run automatically after a power cycle.