Output Controls

Overview

Output controls are interactive dashboard widgets that send data back to a connected device. While standard widgets visualize incoming telemetry, output controls transmit commands, setpoints, and parameters from the Serial Studio dashboard, so the dashboard can both read from and write to the device.

Each output control uses a user-defined JavaScript transmit(value) function that converts widget interactions (button clicks, slider drags, text input) into the exact bytes your device expects. That makes output controls protocol-agnostic: the same slider widget can drive a plain-text serial command, a JSON payload, or a binary packet by changing only the transmit function.

Output controls require a Pro license.

How Output Controls Work

flowchart LR
    A["User Interaction<br/>(click, drag, type)"] --> B["Widget passes value<br/>to transmit(value)"]
    B --> C["JavaScript returns<br/>formatted command"]
    C --> D["Serial Studio sends<br/>bytes to device"]

1. The user interacts with a control on the dashboard (clicks a button, moves a slider, types text, etc.).
2. The widget calls its JavaScript transmit(value) function with the interaction value.
3. The function returns a string (binary payloads are byte-strings built with String.fromCharCode).
4. Serial Studio transmits the result to the connected device.

Transmission is rate-limited to a minimum of 50 ms between sends, preventing device buffer overflows during continuous interactions like slider drags.

Output Control Types

Button

Sends a single command on click.

Slider

Sends a numeric value from a draggable slider.

The value passed to transmit() is a number clamped to [Min, Max]. Transmissions occur continuously while dragging, rate-limited to 50 ms intervals.

Toggle

Binary on/off switch.

Passes 1 to transmit() when switched on, 0 when switched off.

Text Field

Accepts arbitrary typed input and sends it as a string.

Knob

Rotary dial for continuous setpoint adjustment. Same numeric properties as Slider (Min, Max, Step, Initial Value) but displayed as a circular dial.

Creating Output Controls

1. Open the Project Editor (toolbar wrench icon).
2. Click one of the Add Output buttons in the toolbar (Button, Slider, Toggle, Text Field, or Knob).
3. An Output Panel group is created automatically if one does not exist.
4. Select the new control in the tree view to configure its properties and transmit function.

Output controls live inside Output Panel groups. You can also add an Output Panel group first (via the toolbar), then add controls to it. Each Output Panel can hold multiple controls of mixed types, packed automatically into as many columns as fit the available width.

The Transmit Function

Every output control has a JavaScript transmit(value) function that defines how interactions become device commands. The function is compiled once when the dashboard opens and executed on each interaction.

Writing a Transmit Function

The function receives a single value parameter and must return a string:

function transmit(value) {
  // value is:
  //   1           for Button clicks
  //   0 or 1      for Toggle state changes
  //   number       for Slider and Knob
  //   "string"    for TextField input

  return "CMD " + value + "\r\n";
}

The return value must be a string. For binary protocols, build a byte-string with String.fromCharCode(...) (see the Binary Packet template); returning a plain array of numbers transmits nothing. Payloads are capped at 65536 bytes, and a transmit() call that runs longer than 500 ms is stopped by a watchdog. Both conditions abort the transmission and flash a red border on the control; hover the control to see the error message.

Built-in Templates

The code editor includes a set of ready-to-use templates (Simple Command, JSON Command, Binary Packet, PWM Control, PID Setpoint, Relay Toggle, AT Command, Modbus Write, CAN Bus Frame, G-Code, GRBL, NMEA, SCPI, SLCAN, and a default starting point). Select one from the template dropdown and customize it for your device.

Simple Command

Sends plain text with a line terminator. Adapts to the widget type automatically.

function transmit(value) {
  if (typeof value === "string")
    return value + "\r\n";

  if (value === 1)
    return "ON\r\n";

  if (value === 0)
    return "OFF\r\n";

  return "SET " + value + "\r\n";
}

JSON Command

Sends structured JSON objects. Useful for firmware that parses JSON input.

function transmit(value) {
  var obj = {
    cmd: "set",
    value: value
  };
  return JSON.stringify(obj) + "\n";
}

Binary Packet

Sends framed binary data with STX/ETX delimiters.

function transmit(value) {
  var STX = String.fromCharCode(0x02);
  var ETX = String.fromCharCode(0x03);
  var cmd = String.fromCharCode(0x01);
  var val = String.fromCharCode(Math.round(value) & 0xFF);
  return STX + cmd + val + ETX;
}

PWM Control

Sends a duty cycle value (0-255) for motor speed, LED brightness, or heater control.

function transmit(value) {
  var duty = Math.round(Math.max(0, Math.min(255, value)));
  return "PWM " + duty + "\r\n";
}

PID Setpoint

Sends a floating-point setpoint with 2 decimal places for PID controllers.

function transmit(value) {
  return "SP " + Number(value).toFixed(2) + "\r\n";
}

Relay Toggle

Sends distinct ON/OFF commands for relay or digital output control.

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

AT Command

Sends AT-style commands for modems, Bluetooth modules, and WiFi modules.

function transmit(value) {
  if (typeof value === "string" && value.length > 0)
    return "AT+" + value + "\r\n";

  return "AT\r\n";
}

Modbus Register Write

Writes a slider value directly to a Modbus holding register using the built-in helper function.

function transmit(value) {
  return modbusWriteRegister(0x0001, value);
}

CAN Bus Frame

Sends a numeric value as a CAN frame using the built-in helper function.

function transmit(value) {
  return canSendValue(0x100, value, 2);
}

Importing from File

Click the import button in the code editor toolbar to load a .js file from disk. This is useful for sharing transmit functions across projects or version-controlling them separately.

Protocol Helper Functions

Every output widget's JavaScript engine includes built-in helper functions for Modbus and CAN Bus protocols. These handle binary byte-packing so you don't have to construct raw bytes manually.

Modbus Helpers

modbusWriteRegister(address, value)

Writes a 16-bit integer to a single holding register.

function transmit(value) {
  return modbusWriteRegister(0x0001, value);
}

modbusWriteCoil(address, on)

Writes a coil value (ON = 0xFF00, OFF = 0x0000).

// Toggle widget controlling a relay coil
function transmit(value) {
  return modbusWriteCoil(0x0000, value);
}

modbusWriteFloat(address, value)

Writes an IEEE-754 32-bit float across two consecutive holding registers (big-endian).

// Slider writing a temperature setpoint as a 32-bit float
function transmit(value) {
  return modbusWriteFloat(0x0010, value);
}

CAN Bus Helpers

canSendFrame(id, payload)

Sends an arbitrary CAN frame with the given identifier and payload.

// Button sending a fixed command frame
function transmit(value) {
  return canSendFrame(0x200, [0x01, 0x00, 0xFF]);
}

// Slider packing its value into a 3-byte payload
function transmit(value) {
  var v = Math.round(value);
  return canSendFrame(0x100, [0x01, (v >> 8) & 0xFF, v & 0xFF]);
}

canSendValue(id, value, bytes)

Sends a numeric value packed big-endian into a CAN frame.

// Slider sending a 16-bit value on CAN ID 0x100
function transmit(value) {
  return canSendValue(0x100, value, 2);
}

// Knob sending a 32-bit value on CAN ID 0x300
function transmit(value) {
  return canSendValue(0x300, value, 4);
}

Combining Helpers with Custom Logic

The helpers return strings that can be concatenated or conditionally selected:

// Write different registers based on a toggle state
function transmit(value) {
  if (value)
    return modbusWriteRegister(0x0010, 1);  // Enable
  else
    return modbusWriteRegister(0x0010, 0);  // Disable
}

// Send a CAN frame with a header byte and the widget value
function transmit(value) {
  return canSendFrame(0x150, [0xAA, Math.round(value) & 0xFF]);
}

Output Panel Layout

Output controls are displayed in an Output Panel widget on the dashboard. The panel uses an adaptive layout engine:

• Controls are packed automatically into as many columns as fit the available width, derived from each control's minimum width.
• Small controls (Button, Slider, Toggle, TextField) stack vertically within columns.
• Tall controls (Knob) span the full column height.
• If controls overflow the visible area, the panel scrolls vertically.

Multi-Source Projects

In projects with multiple data sources (devices), the target device is determined by the source of the Output Panel group a control belongs to, not by a per-control property. Every control in a group transmits to that group's source. To send to a different device, place the control in an Output Panel group assigned to that source.

Output Controls vs. Actions

Both output controls and Actions send data to connected devices, but they serve different purposes:

Use Actions for simple fire-and-forget commands, periodic polling, and auto-execute-on-connect sequences. Use Output Controls when you need interactive controls with continuous values, custom data formatting, or a mix of widget types.

Examples

Motor Speed Controller

Control motor speed with a slider and an emergency stop button.

Speed transmit function:

function transmit(value) {
  return "SPD " + Math.round(value) + "\r\n";
}

Emergency Stop transmit function:

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

Relay Control Panel

Toggle 3 relays independently.

Each relay uses a customized transmit function with its relay number:

// Relay 1
function transmit(value) {
  return value ? "R1 ON\r\n" : "R1 OFF\r\n";
}

Sensor Calibration Interface

Combine a text field for commands with a knob for fine adjustment.

Command transmit function:

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

Offset transmit function:

function transmit(value) {
  return "OFFSET " + Number(value).toFixed(1) + "\r\n";
}

Modbus PID Controller

Control a PID loop over Modbus by writing setpoint, Kp, and enable/disable to holding registers.

Setpoint transmit function (32-bit float to registers 0x0010–0x0011):

function transmit(value) {
  return modbusWriteFloat(0x0010, value);
}

Kp Gain transmit function (32-bit float to registers 0x0012–0x0013):

function transmit(value) {
  return modbusWriteFloat(0x0012, value);
}

Enable transmit function (coil at address 0x0000):

function transmit(value) {
  return modbusWriteCoil(0x0000, value);
}

CAN Bus Motor Controller

Control a motor over CAN Bus with speed setpoint and emergency stop.

Speed transmit function (16-bit value on CAN ID 0x100):

function transmit(value) {
  return canSendValue(0x100, value, 2);
}

Direction transmit function (single byte on CAN ID 0x101):

function transmit(value) {
  return canSendFrame(0x101, [value ? 0x01 : 0x00]);
}

E-Stop transmit function (fixed command frame on CAN ID 0x1FF):

function transmit(value) {
  return canSendFrame(0x1FF, [0xFF, 0x00]);
}

Common Mistakes

Controls Do Not Appear on Dashboard

Symptom: Output controls are configured in the Project Editor but do not appear on the dashboard.

Fix: Ensure the device is connected. Output panels only appear on the dashboard while a connection is active. Also verify that the controls are inside an Output Panel group (group type must be "Output").

Commands Not Received by Device

Symptom: The control is visible and interactive, but the device does not respond.

Fix:

1. Check the Console view to confirm data is being sent.
2. Verify the transmit function returns a properly terminated string (most devices expect \r\n).
3. In multi-source projects, confirm the control is in an Output Panel group assigned to the correct device.
4. Check that your Pro license is active. Transmission is disabled without it.

Slider Sends Too Many Commands

Symptom: The device is overwhelmed or the serial buffer overflows while dragging a slider.

Fix: The built-in 50 ms rate limit prevents most flooding, but if your device needs more time between commands, increase the step size to reduce the number of discrete values, or add debouncing logic in your transmit function.

Transmit Function Error

Symptom: The control shows the red text "No transmit function defined" instead of its widget.

Fix: Open the Project Editor and check the transmit function for syntax errors. The function must be a valid JavaScript function named transmit that accepts one parameter and returns a string. This label also appears when the field is left empty. Runtime errors raised during a transmit (watchdog timeout or an oversize payload) abort the send but are not shown on the widget; use the Console view to confirm what was sent.

Tips

• Start with a built-in template and modify it. That avoids common syntax mistakes.
• Test with the Console view open to see exactly what bytes are being transmitted.
• Combine output controls with input widgets in the same dashboard for full closed-loop monitoring (e.g., a slider to set a target temperature alongside a gauge showing the actual temperature).
• Use the built-in protocol helpers (modbusWriteRegister, canSendFrame, and so on) instead of packing binary bytes by hand. See Protocol Helper Functions above.
• For protocols beyond the built-in helpers, define your own helper functions next to transmit() in the same script. Variables declared outside transmit() persist across calls.

See Also

• SDK Reference: the protocol encoders (modbusWriteRegister, canSendFrame, ...) a transmit() function returns, and the wider scripting surface.
• Actions: simple command buttons with timer support.
• Project Editor: full guide to creating and configuring projects.
• Toolbar & Button Reference: the Project Editor's add-control buttons and the rest of the app's chrome.
• Widget Reference: all input and visualization widget types.
• Frame Parser Scripting: Lua and JavaScript parser reference.
• Data Sources: configuring device connections.