Gas concentration data is available on the RS485 digital communication channel. The RS485 channel is 2-wire.
Connection is via the TX, RX and DNG screw terminal connectors on the sensor board.
The communication protocol for the RS485 serial interface is specified below.
These command protocols are specified by Aeroqual Limited, all rights reserved. Aeroqual reserves the right to change the protocol without notification.
Communication commands
For data format and representations, see the section Data value format representation.
Comma and spaces are not applied for every command and reply data stream - they are just used for clearly specifying data stream.
Aeroqual SM 70 sensor module RS485 protocol is a slave mode. Master receivers need send request command to get response.
SM70 sensor data request command, 4 bytes
Command: BASE, DATA_REQUEST, RESERVED, CHECKSUM
Example: 0x55, 0x1A, 0x00, 0x91
Reply data stream is 15 byte.
Reply: SENSOR, DATA_REPORT, DATA1, DATA2, RESERVED, STATUS1, STATUS2, CHECKSUM
The second byte (DATA REPORT) will be either 0x1A or 0x0F or 0x10. Only if DATA REPORT = 0x10 will DATA1 be a valid concentration reading.
- SENSOR - 1 byte monitor reply data stream header, see section 2 for its value.
- DATA_REQUEST - 1 byte heater data report, see section 2 for its value.
- DATA_REPORT - 1 byte gas concentration data report command, see section 2 for its value.
- DATA1 - 4 bytes floating point data, when command reply is DATA_REPORT, this value is gas concentration in ppm.
- DATA2 - reserved
- RESERVED - is 2 bytes data space reserved.
- STATUS1 - 1 byte monitor and sensor status indication, refer section 3 for details.
- STATUS2 - 1 byte reserved.
- CHECKSUM - 1 byte the data stream's check sum - that makes the command stream total sum is zero.
SM70 sensor information request command
Command: BASE, SENSOR_INFO, RESERVED, CHECKSUM
Reply: SENSOR, SENSOR_INFO, VERSION_NO, DISPLAY, NMAE_LENGTH, SENSOR_NAME, RESERVED, CHECKSUM
- BASE - 1 byte information request command header, see section 2 for its value.
- SENSOR - 1 byte monitor reply data stream header, see section 2 for its value.
- SENSOR_INFO - 1 byte command see section 2 for its value
- VERSION_NO - 1 byte sensor version number, see section 2 for its value.
- DISPLAY - 1 byte, gas concentration value display format type, see section 2 for its value.
- NAME_LENGTH - 1 byte specify the sensor name byte length
- SENSOR_NAME - 7 bytes, the gas sensor name ASCII code, its valid bytes are specified by NMAE_LENGTH
- RESERVED - 1 byte
- CHECKSUM - 1 byte the data stream's check sum - that makes the command stream total sum is zero.
Protocol command values
Protocol command values are in hexadecimals not ASCII.
BASE = 0x55 //header command used for receiver command
SENSOR = 0xAA //header command used for monitor reply
DATA_REPORT = 0x10 //regular data report command
DATA_REQUEST = 0x1A //heater data request/report command
SENSOR_INFO = 0xFB //parameters upload command
RESERVED = 0x00 //the byte not been used for information transfer
CHECKSUM * a data stream's check sum - that makes the command stream total sum is zero.
DISPLAY * display format can be following:
- = 0x01 - 1 digit int, 3 decimal points, eg. 0.500 ppm
- = 0x02 - 2 digits int, 2 decimal points, eg. 12.20 ppm
- = 0x03 - 3 digits int, 1 decimal point, eg. 126.8 ppm
- = 0x04 - 4 digits int, no decimal point, eg. 2888 ppm
STATUS1 * 8 bits monitor and sensor status information
SS0 * b0 \ 00 sensor working fine
SS1 * b1 / 01 sensor failure
11 sensor aging
- Reserved1 * b2 Reserved not been used
- Reserved2 * b3 Reserved not been used
- Reserved3 * b4 Reserved not been used
- Reserved4 * b5 Reserved not been used
- Reserved5 * b6 Reserved not been used
- Reserved6 * b7 Reserved not been used
Data value format representation
The floating point data values use IEEE754 32 bits floating point little ending representation. They are:
DATA1, DATA2
Data transfer mechanism
Due to the monitor main chips feature, 4 bytes floating point data and 2 bytes int data send sequence are low byte first, high byte last, such as Data value format representation data DATA1 and FACTOR.
Communication settings
RS485 communication port settings:
- Baud rate: 4800
- Data bits: 8
- Stop bits: 1
- Parity: none
- Flow control: none