SenTec SEM228P PAR Sensor — Web Serial Client
Reads the photosynthetically active radiation value and configures the calibration deviation, baud rate and device address of a ChengDu SenTec SEM228P sensor directly from the browser, over Modbus RTU / Web Serial. Applies to the RS485 (Modbus-RTU) variant — the 4–20 mA / 0–5 V / 0–10 V analog variants have no digital interface.
Register map per SEM228P PAR Sensor Operation Manual V2208R · cdsentec.com
Web Serial API not available in this browser. Use a Chromium-based browser
(Chrome, Edge, Opera) on desktop.
1. Connection
DISCONNECTED
Factory defaults: address 1, 4800 baud, 8 data bits, no parity, 1 stop bit.
Valid addresses are 1–254. Wiring: brown = V+ (7–30 V DC), black = V−, green = 485-A, blue = 485-B.
Reads registers 0x07D0–0x07D1 through the manual's universal address
0xFF and reports the address
and baud rate the device answers with. The current connection baud rate still has to match the device, so try
2400 / 4800 / 9600 in turn if it times out. Only reliable with exactly one device on the bus.
2. Live PAR Reading
—
µmol/m²·s
| Quantity | Raw register | Value |
|---|---|---|
| Connect, then "Read Once" or start auto-polling. | ||
Register 0x0000 is read-only and already carries the actual value in µmol/m²·s (resolution
1). Rated measuring range is 0–2500; accuracy ±2 % at 1000 µmol/m²·s, 25 °C.
A constant 0 usually means no light or a protective cover still fitted on the dome.
Reading History
Irradiance estimate (W/m²)
Applies
W/m² = PAR / factor. 4.57 µmol/J is the usual figure for daylight; LED and
HPS spectra need their own factor. This is a display-only estimate — the sensor measures photon flux, and
nothing is written to the device.
3. Calibration Deviation
Register 0x0052 holds a signed 16-bit offset added to the measured value, in whole
µmol/m²·s. Use it to trim the sensor against a reference instrument; write 0 to remove any
correction.
4. Device Configuration
A new baud rate takes effect on the device immediately after the write. If you targeted the
currently connected slave address, this tool updates Section 1 to match and disconnects so you can reconnect
on the new settings. Only use the universal address 0xFF when a single device is on the bus — every listening
device would otherwise adopt the same address.
Register map
| Address | Content | Access | Range |
|---|---|---|---|
| 0x0000 | Photosynthetically active radiation | read only | 0–2500 µmol/m²·s |
| 0x0052 | Deviation (offset) | read / write | 16-bit signed |
| 0x07D0 | Device address | read / write | 1–254 (default 1) |
| 0x07D1 | Baud rate | read / write | 0 = 2400, 1 = 4800, 2 = 9600 |