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.
The Hawk is programmed to automatically interface with Measurement (M!) commands from SDI-12 sensors. See "Configure SDI-12 Readings" below for more information. If you need to read alternate measurements from SDI-12 sensors such as Data commands (D!) or Verification commands (V!) please contact Digital Matter support.
What SDI-12 Commands does the Hawk Use?
The Hawk uses the M! command. It is mainly designed around the use-case of monitoring soil moisture probes. Implementing multiple arbitrary commands in a specific sequence generically is more complicated - but contact us if you have specific requirements to discuss the options.
The full control flow of the SDI-12 measurement, for an address a, and measurement type t, is:
Tx: aI! (identify device)
Rx: axxxxx…. The response gets sent as part of the SDI12 data. If no response, then we give up this measurement.
For each measurement parameter, the following goes through each set bit in the mask
Tx: aM! (or aMt! if the type is not 0)
Rx: aXXXn. XXX is how long we need to wait before the samples are ready, and n is how many it will return (both can be 0).
After waiting XXX seconds, the following goes through each value of n, from 0 to 9, until either 0 data is returned (but still a response), or we reach 15 individual values returned. Each aDn command can return more than one value
Tx: aDn
Rx: a<data>
If at any time no response is heard, the measurement is aborted.
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
