The SDI-12 interface is a low data rate protocol designed for agricultural and scientific sensors (SDI-12 Website).
A soil moisture probe will often have multiple sensors along its length. Each sensor can have an address between 0 and 9 (inclusive), and allows for up to 10 different 'measurements'. Each of these measurements can actually return a number of different values (up to 15). For example, a soil moisture probe may have a soil moisture and soil temperature measurements, and each of these will return a number of values, each being taken at a certain point along the probe.
Configure SDI-12 Readings
Configure Task
Configure SDI-12 Action 1 or 2 as an action for a task. The below task is for an SDI-12 reading logged + uploaded hourly.
Configure Vboost Voltage Level
While the SDI-12 bus operates at 5V, most SDI-12 probes require 12V to power the sensors.
We need to tell the Hawk to output 12V via the Ag-Tech1 boost output.
Configure SDI-Reading
We specify
- SDI 12 Address
- Address of the probe to read
- Log Identification/Use Old Data Field
SDI-12 data is sent in the following fields. Field 34 is new to the Hawk.
If we only have 1 SDI-12 measurement we are taking, from 1 address, it is not necessary to identify this in the payload. So to save data we can select Log Identification as No.
In general the default Log Identification = No and Use old Data Field = No should be used for most applications.
FID = 4: SDI-12 Device Identification
Length = N + 1
Offset
Data Type
Length
Description
Unit
0
BYTE
1
Device Address
(SDI12 single char address)
1
BYTE
N
SDI12 Identification string
(typically 30 ASCII characters)
FID = 5: SDI-12 Measurement
Length = 1 + N*4
The SDI12 spec allows for a device to support 10 different types of measurements (0-9).
Offset
Data Type
Length
Description
Unit
0
BYTE
1
SDI12 Measurement Type (0-9)
1 + N*4
INT32
4
Measurement (scaled by 1000)
<n data values follow>
Scaled by 1000
Eg milli-degrees centigrade
FID = 34: SDI-12 Measurement 2
Length =3 + N * X
This field replaces FID 4 on newer devices.
The SDI12 spec allows for a device to support 10 different types of measurements (0-9).
Offset
Data Type
Length
Description
Unit
0
BYTE
1
Device Address
(SDI12 single char address)
1
BYTE
1
SDI12 Measurement Type (0-9)
2
BYTE
1
SDI12 Data Format
0x00 = Scaled by 1000, stored in INT32
Other values are reserved.
3
Dependant on format
N * X
N measurements of size X, specified by the format.
Log Identification Use Old Data Field Fields Sent No No FID 34 No Yes FID 5 Yes No FID 4 + FID 34 Yes Yes FID 4 + FID 5 - Power on Delay
- Most sensors require some time after power is applied to 'warm-up' or settle down before the reading is taken.
- Run Measurement N
- Select from the dropdowns which measurements we would like to run
- Sensor Address 0
- Use the new data field, with no ID field
- Power on delay 15 seconds
- Measurements 1, 4 and 7
Example
