SDI-12

Key takeaways

SDI-12 belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect SDI-12 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of SDI-12 from memory before moving on to harder problems.

Reference excerpt

SDI-12 (Serial Digital Interface at 1200 baud) is an asynchronous serial communications protocol for intelligent sensors that monitor environment data. These instruments are typically low-power, are used at remote locations, and usually communicate with a data logger or other data acquisition device. The protocol follows a client-server configuration whereby a data logger (SDI-12 recorder) requests data from the intelligent sensors (SDI-12 sensors), each identified with a unique address.

History The first version of SDI-12 was released in 1988. It was designed by a coalition which included the U.S. Geological Survey's Hydrologic Instrumentation Facility (HIF) and a group of private companies. The SDI-12 Specification is maintained by a non-profit corporation called the SDI-12 Support Group. The most recent upgrade (Version 1.4) was made in July 2016. The most recent version of the SDI-12 Specification (still version 1.4, with clarifications), was released on January 30, 2021.

Versions Version 1.0 was released in 1988. Version 1.1 was released in 1994. It was a rewriting and clarification of the version 1.0 specification under contract by Michael Jablonski's NR Systems (dba Natural Resources Consulting) and Campbell Scientific. Version 1.2 was released in April 1996. It defines:

Concurrent measurement command (aC!) Address query command (?!) Change address command (aAb!) Continuous measurement command (aR[0-9]!) Version 1.3 was released in April 2000 and defines:

Cyclic Redundancy Check (CRC) for data validation Electrical interface characteristics such as number of conductors, voltage logic levels and transitions, and line impedance. Communications protocol such as means of establishing contact with a specific address (sensor), the measurement command set, sensor response behavior, byte frame format and allowed characters. Timing requirements such as length of the break condition used to awaken sensors, minimum time between messages and length of time before a sensor enters a low-power state. Version 1.4 was released in July 2016 and defines:

High volume commands (aHA! for ASCII and aHB! for binary format) which allow up to 999 parameters to be returned from a sensor. Metadata Commands (aIM!) which provide meta data information about the measurement, such as the parameter name and units.

Advantages The specification document describes a number of advantages including

Interchangeability of sensors without reprogramming of data acquisition devices Power is supplied to sensors through the interface Ability to implement self-calibration algorithms within the sensor itself and use low-cost EEPROMs for information storage Applicability of training in SDI-12 to a variety of sensors and data recorders The SDI-12 Specification is in the public domain.

Drawbacks Specific electrical interface (serial data line, ground line, and 12-volt line) Baud rate limited to 1200

Details Communication occurs over a single data line in half-duplex. The digital addressing system allows an SDI-Recorder to communicate with up to 62 individual sensors. Only the pre-configured sensor matching that address will respond (handshake). Other sensors on the same line will not respond until called and typically stay in "sleep mode" (low power mode), until called. Electrically the protocol is a three wire digital connection: data, ground and 12 V. The data signal, using 5V logic levels, is similar to RS-232 with the same asynchronous byte encoding. The inline data is human readable as the data is transmitted in ASCII. All SDI-12 communications are transmitted in ASCII at 1200 baud with 7 data bits and an even parity bit. The standard also specifies a communications protocol that allows sensors to remain in a low-power sleep state until awoken by a serial break signal sent by the master. The first character of each command is a unique sensor address that specifies with which sensor the recorder wants to communicate. Other sensors on the SDI-12 bus ignore the command and return to low-power standby mode. The protocol also specifies a retry mechanism to overcome any data corruption. CRCs were introduced to the SDI-12 protocol with release of version 1.3. The standard provides guidelines on transient protection and does not require the use of a specific connector.

Applications The protocol is most commonly supported by hydrographic sensors (level and flow), water quality sensors, meteorological instruments, soil moisture and plant physiology. Less common but also known to be supported are sensors for the measurement of solar radiation, tilt (inclinometers), and other geophysical parameters.

See also Data logger RS-232 RS-485 Serial communications Transducers 1Wire

References

External links SDI-12 Support Group

Worked examples

Example 1 — a first encounter with SDI-12

Start with the simplest possible case. Write down what SDI-12 claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to SDI-12 before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about SDI-12 ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of SDI-12

In research: SDI-12 appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses SDI-12 in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: SDI-12 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Data transmission, so understanding it makes those chapters shorter.
In everyday life: Look for SDI-12 outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study SDI-12 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what SDI-12 means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain SDI-12 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is SDI-12 in simple terms?

SDI-12 (Serial Digital Interface at 1200 baud) is an asynchronous serial communications protocol for intelligent sensors that monitor environment data. These instruments are typically low-power, are used at remote locations, and usually communicate with a data logger or other data acquisition devic…

Why does SDI-12 matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study SDI-12?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on SDI-12.