Testo T549i Pressure Probe

This Serial Studio project turns a Testo T549i high-pressure smart probe into a live dashboard over Bluetooth Low Energy. It is a complete tour of Serial Studio's automation stack in one small project: a Control Loop wakes the probe, a binary frame parser decodes the raw stream, and per-dataset Lua transforms derive the engineering units.

What it does

• Restores the probe's GATT configuration from the project: service 0xFFF0, notify characteristic 0xFFF2, both saved by UUID.
• Sends the vendor "enable measurement" handshake to write characteristic 0xFFF1 from the project's Control Loop, so the probe starts streaming without any phone app.
• Decodes the probe's binary notifications with a JavaScript frame parser that emits raw device units only: differential pressure in pascal and battery level in percent.
• Converts pascal into psi with a one-line Lua transform, and shows the result on a plot, a meter, and a battery bar.

The BLE protocol

The probe exposes a vendor GATT service with a split read/write pair:

The probe only streams after a three-command enable sequence is written to 0xFFF1:

56 00 03 00 00 00 0C 69 02 3E 81
20 00 00 00 00 00 07 7B
11 00 00 00 00 00 03 5A

Measurements then arrive on 0xFFF2 as name-prefixed binary frames, interleaved with short checksum and status notifications:

Battery level arrives the same way, with the field name BatteryLevel.

Control Loop: the wake-up handshake

The Control Loop runs setup() once when the device connects and loop() while it stays connected, like an Arduino sketch. Serial Studio reports the source as connected only after the saved service and notify characteristic are wired, so the script can write immediately. The handshake, condensed from the project's script:

const WRITE_UUID  = "fff1";
const ENABLE_CMDS = ["5600030000000C69023E81", "200000000000077B", "110000000000035A"];

function setup() {
  for (var i = 0; i < ENABLE_CMDS.length; ++i) {
    var r = io.ble.writeCharacteristic(WRITE_UUID, ENABLE_CMDS[i], SerialStudio.Hex);
    if (!r.ok)
      return;

    delay(100);
  }
}

The full script in the project keeps a sent flag and retries from loop() if a write fails.

Frame parser: minimal raw decode

The parser does one job: find the field name in each notification and read the IEEE-754 float that follows it. It emits raw device units and nothing else; notifications without a marker just re-emit the held values, so every dataset updates on a steady timeline.

parse(frame) -> [ pressure in Pa, battery in % ]

No unit conversion happens in the parser. That keeps it a pure description of the wire format, and it never needs to change when you want another unit on the dashboard.

Engineering units with transforms

The pressure dataset reads parser channel 1 and converts pascal to psi in its Lua transform:

Adding another unit (bar, kPa, inHg) is one more dataset with a one-line transform that reads the same channel; the parser stays untouched. The meter spans 0 to 1000 psi, and the math.max clamp pins negative sensor drift to zero.

Project configuration

Setup

1. Open the project and select Bluetooth LE as the input source.
2. Power on the probe and pick it from the device list (it advertises as T549i SN:...).
3. Click Connect. Serial Studio restores the service and notify characteristic, the Control Loop sends the enable sequence, and readings start streaming.
4. Clamp the probe on a pressure port and watch the meter. Unpressurized, it reads zero; the transform clamps negative sensor drift.