Output Widgets: JavaScript "transmit function" API

Output widgets (Pro) translate UI state into bytes the device receives. Each widget instance has one user-authored JS function that runs on every state change (button press, slider release, toggle flip, textfield submit).

Contract

The entry point is transmit, not output / send. It takes a single scalar value and returns the bytes to send:

function transmit(value) {
  return "CMD " + value + "\r\n";
}

value is a single scalar whose type depends on the widget kind — there is no state object:

Return either:

• a String: encoded using the widget's configured TX encoding and sent. For binary payloads build a byte-string with String.fromCharCode(...) the way the bundled modbus/can helpers do — plain Array<number> returns are dropped (they convert to an empty byte array),
• null / undefined / an empty result: nothing is sent.

There is a maximum payload size; oversized returns are dropped with an error.

Injected protocol helpers (globals)

The Modbus/CAN helpers below are injected as globals, alongside the full host API: apiCall, tableGet/tableSet, datasetGetRaw/datasetGetFinal, deviceWrite, actionFire, notify*, the dashboard helpers, and the generated SerialStudio SDK. Protocol logic beyond these helpers you build yourself in the function body (see the bundled reference scripts below — they self-contain their protocol logic, they are NOT globals).

modbusWriteRegister(address, value)  // 16-bit holding register -> 4-byte payload
modbusWriteCoil(address, on)         // coil ON=0xFF00 / OFF=0x0000 -> 4-byte payload
modbusWriteFloat(address, value)     // IEEE-754 float over two registers -> 6-byte payload
canSendFrame(id, payload)            // payload = string or Array<number> -> [id_hi,id_lo,dlc,...data]
canSendValue(id, value, bytes = 2)   // big-endian integer over canSendFrame

These produce raw binary payloads (returned as strings). There is no crc16 / nmea / slcan / gcode / scpi / packet global — those names do not exist. Author that logic inline or adapt a bundled reference script.

Bundled reference scripts

scripts.list{kind: "output_widget_js"} enumerates ready-to-adapt examples; scripts.get{kind: "output_widget_js", id} returns the full source. Available templates include: at_command, binary_packet, canbus_frame, gcode_command, grbl_command, json_command, modbus_write, nmea_sentence, pid_setpoint, pwm_control, relay_toggle, scpi_command, simple_command, slcan_command. Each defines its own transmit(value) and computes any checksum/framing inline — adapt one instead of writing protocol code from scratch.

Examples

Toggle a relay (textual command)

function transmit(value) {
  return value ? "RELAY ON\r\n" : "RELAY OFF\r\n";
}

Slider value over Modbus (built-in helper)

function transmit(value) {
  return modbusWriteRegister(0x0010, Math.round(value));
}

NMEA sentence with inline XOR checksum (no helper exists)

function transmit(value) {
  var body = (typeof value === "string") ? value : "GPCMD," + value;
  var chk = 0;
  for (var i = 0; i < body.length; i++)
    chk ^= body.charCodeAt(i);

  var hex = ("0" + chk.toString(16)).slice(-2).toUpperCase();
  return "$" + body + "*" + hex + "\r\n";
}

Slider over CAN (built-in helper)

function transmit(value) {
  return canSendValue(0x123, Math.round(value), 2);
}

Validate before applying

Compile and test the function without touching the live widget:

assistant.script.dryRun{kind: "output_widget", code, inputValue?, hex?}

It compiles transmit() in the same injected-helper + table-API environment the live widget uses and returns {ok, compileError?, line?}. Pass inputValue (and hex: true to treat it as hex bytes) to also run transmit() once and get sampleRun.outputHex + byteCount.

Sandbox

Same isolation as parser scripts — no network, no filesystem, no Qt access. Transmit scripts run on UI events, not on the frame hot path, so allocations are not a concern.