UART protocol

The FDO2-G2's ASCII command protocol as the driver uses it: framing, commands, reply fields, status bits, error codes and the optional CRC.

The source is section 4 of the FDO2-G2 datasheet (v5, 03/2026). Other PyroScience instruments use a different command set; this page and the driver apply to the FDO2-G2 only.

Framing

• A command is a header (# and capital letters, such as #MOXY), optional values separated by spaces, and a CR (0x0D) or CR LF.
• A reply that worked echoes the header, appends the requested values and ends with a single CR.
• A reply that failed is #ERRO E with a negative code E.
• Numbers are decimal ASCII in the range −2147483648 to 2147483647. The only exception is the #IDNR value, an unsigned 64-bit number of up to 20 digits.

The driver sends the command, waits for one line, ignores a LF if one appears, and splits the line at spaces. It accepts a reply only if it ends with the CR, the header matches the command exactly, the number of values is the one listed below, and every value is a decimal number in its range: signed 32-bit, unsigned 32-bit for the status S, unsigned 64-bit for #IDNR. Anything else is DriverError::ProtocolError.

Commands the driver sends

Command	Reply	Driver call
#VERS	#VERS D N R S	ReadVersion()
#IDNR	#IDNR N	ReadUniqueId()
#MOXY	#MOXY O T S	MeasureMoxy(), or SendMeasure(Measure::Moxy)
#MRAW	#MRAW O T S D I A P H	MeasureMraw(), or SendMeasure(Measure::Mraw)
#LOGO	#LOGO	FlashLogo(): the status LED flashes 4 times, about 1 s, then the echo arrives

#VERS reports D = 8 for every FDO2-G2, N = 1 oxygen channel, the firmware revision as R = 357 for 3.57, and S, a bit mask of the sensors present (bit 0 oxygen, bit 1 temperature, bit 2 pressure, bit 3 humidity; 15 means all). The unique id from #IDNR is not the serial number printed on the device.

Reply fields

Every field is a signed 32-bit integer, except the status S, which is unsigned. The driver keeps the raw value and adds the scaled one.

Field	Raw unit	Scaled member	Typical range
O oxygen partial pressure	10−3 hPa	p_o2_hpa	0 to 210 hPa in air
T sensor temperature	m°C	temp_c	0 °C to 50 °C
S status	bits	status_u32	0 or 1 in normal operation
D phase shift dphi	m°	dphi_deg	5° to 60°
I signal intensity	µV	signal_intensity_mv	100 mV to 500 mV
A ambient light	µV	ambient_light_mv	0 to about 100 mV
P pressure at the back of the housing	µbar	pressure_mbar	900 mbar to 1100 mbar
H humidity inside the housing	m%RH	rh_in_housing_pct	10 %RH to 90 %RH

O, T and S come with both #MOXY and #MRAW; the rest only with #MRAW. Every scaled member is the raw value times 10⁻³.

Two rules from the datasheet for the #MRAW values:

• Signal and ambient light. Keep I + A under about 2000 mV. Above it the detector can saturate; shade the sensor. Compare signal intensities only at similar oxygen levels: about 200 mV to 400 mV is typical at 200 hPa and room temperature.
• Volume percent. When the gas at the membrane is at the same pressure as the back of the housing, % O₂ = 100 x pO₂ / P. fdo2::VolumePercentO2(p_o2_hpa, pressure_mbar) computes it (1 mbar = 1 hPa) and returns 0 for a pressure of zero or below.

Status bits

Bit	Constant in fdo2::moxy_status	Meaning
0	kWarnAutoAmpReduced	Warning: amplification reduced to avoid saturation; the reading is valid
1	kFatalSignalTooLow	Fatal: oxygen signal below 20 mV
2	kFatalSignalTooHigh	Fatal: oxygen signal or ambient light too high
3	kFatalRefTooLow	Fatal: reference signal below 20 mV
4	kFatalRefTooHigh	Fatal: reference signal or ambient light above 2400 mV
5	kFatalTempSensor	Fatal: temperature sensor failure
7	kWarnHumidityHigh	Warning: humidity in the housing above 90 %RH
9	kErrPressureSensor	Error: housing pressure sensor failure; pO2 not affected
10	kErrHumiditySensor	Error: housing humidity sensor failure; pO2 not affected

Bits 6 and 8 are reserved. The datasheet's rule: under normal operation S is 0 or 1; any other value means O and T are or can be wrong, and the user must check S on every measurement. fdo2::MoxyStatusOkForOxygen(S) is that rule.

Error codes

A command that failed returns #ERRO E. The driver turns it into DriverError::DeviceError and keeps E in LastDeviceErrorCode().

E	Type	Meaning and what to do
-1	General	A non-specific error
-21	UART parse	The command did not parse; repeat it
-22	UART receive	The command was not received correctly (not ready, bad terminator); repeat it
-23	UART header	The header has characters other than A to Z; repeat it
-26	UART request	The header matches no supported command
-60	Invalid calibration	A calibration was out of range and was discarded

Any other code is potentially fatal: the datasheet says to replace the sensor and contact PyroScience.

Optional CRC

#CRCE 1 makes the module end every reply with a colon, a space and a CRC-16 as decimal ASCII before the CR: #MOXY 203456 17892 0: 43291, then CR. The CRC is the MODBUS one (CRC-16-IBM: reflected polynomial 0xA001, initial value 0xFFFF, no final XOR) over every byte from the start of the reply up to, not including, the colon, spaces included. The setting is stored in flash and survives a power cycle.

The driver checks the CRC of every reply that carries one, #ERRO replies included, and removes the suffix before it parses the reply. A wrong checksum returns DriverError::CrcMismatch; a colon not followed by a number from 0 to 65535 returns ProtocolError. Call SetCrcRequired(true) on a module with the CRC on to reject replies that arrive without it. The driver never sends #CRCE. Without the CRC, the datasheet's first-level check is the echo of the command header, which the driver does compare.

Commands the driver does not send

Command	What it does	Why the driver leaves it out
#CALO, #CAHI P	Two-point calibration at 0 % and at P; discards the factory calibration	Writes flash; up to 10 s; needs a calibration gas
#SETM M	Flash duration mode 0 (lowest drift) to 4 (lowest noise); default 2	Writes flash
#BCST T	Broadcast #MRAW replies every T ms (100 to 10000)	Writes flash; interleaves with command replies
#CRCE K	CRC on or off	Writes flash
#BAUD R	Baud rate until the next power cycle	Not implemented
#RDUM, #WRUM	Read and write 64 words of user memory	Not implemented; #WRUM writes flash

The module tolerates about 20,000 flash cycles, and an interrupted flash write can corrupt it. Send these commands from a service tool with a stable supply, not from production firmware. If a module arrives with broadcast mode on, send #BCST 0 once: its broadcasts otherwise interrupt the driver's replies.

Next

API guide uses these commands from C++; Troubleshooting maps errors and status bits to causes.