SDI-12 power considerations
When a command is sent by the data logger to an 
SDI-12 Serial Data Interface at 1200 baud. Communications protocol for transferring data between the data logger and SDI-12 compatible smart sensors. probe, all probes on the same SDI-12 port will wake up. However, only the probe addressed by the data logger will respond. All other probes will remain active until the timeout period expires.
Example:
Probe: Water Content
Power Usage:
- Quiescent: 0.25 mA
- Active: 66 mA
- Measurement: 120 mA
Measurement time: 15 s
Timeout: 15 s
Probes 1, 2, 3, and 4 are connected to SDI-12 port C1.
The time line in the following table shows a 35-second power-usage profile example.
For most applications, total power usage of 318 mA for 15 seconds is not excessive, but if 16 probes were wired to the same SDI-12 port, the resulting power draw would be excessive. Spreading sensors over several SDI-12 terminals helps reduce power consumption.
|
Example power use for a network of SDI-12 probes |
||||||||
|---|---|---|---|---|---|---|---|---|
| Time into measurement processes | Command | All probes awake | Time out expires | Probe 1 (mA) | Probe 2 (mA) | Probe 3 (mA) | Probe 4 (mA) | Total (mA) |
| Sleep | 0.25 | 0.25 | 0.25 | 0.25 | 1 | |||
| 1 | 1M!
|
Yes | 120 | 66 | 66 | 66 | 318 | |
| 2–14 | 120 | 66 | 66 | 66 | 318 | |||
| 15 | Yes | 120 | 66 | 66 | 66 | 318 | ||
| 16 | 1D0!
|
Yes | 66 | 66 | 66 | 66 | 264 | |
| 17-29 | 66 | 66 | 66 | 66 | 264 | |||
| 30 | Yes | 66 | 66 | 66 | 66 | 264 | ||
| Sleep | 0.25 | 0.25 | 0.25 | 0.25 | 1 | |||