SDI-12
Encyclopedia
SDI-12 is the acronym for "Serial Data Interface at 1200 Baud". SDI-12 is an asynchronous, ASCII
, serial communications
protocol
that was developed for intelligent sensory instruments that typically monitor environmental data
. These instruments are typically low-power (12 volt), are often used in remote locations, and usually communicate with a data logger
or other data acquisition
device. In this master-slave
configuration, the data logger
or data acquisition
device typically acts as the master (SDI-12 Recorder and Interogator) to the data monitoring instruments, which are the slaves (SDI-12 sensors). One master can communicate with multiple slaves, so the SDI-12 protocol requires that each device in the serial network be identified with a unique address, which is represented by a single ASCII
character.
This communication is achieved by digital communications along a single serial line.
The digital addressing system allows an SDI-Recorder to send out an address over a single line
that is occupied by up to 62 sensors with only the pre-configured sensor matching that address will respond (handshake), while the other sensors on the same line will not respond until called and typically stay in "sleep mode"(low power mode), until called (often in a sequence) at a later time by the SD-I Recorder (Master).
Advantages of this are the ability to use a single available data channel for many sensors (In many cases a technician may want to set up more sensors but is limited by the number of analogue channels that may be available on a particular Data Logger).
Another popular advantage is the ability to interface otherwise incompatible equipment.
This allows more sensors to be utilised on a limited number of channels, transmit over longer distances and save power.
SDI-12 enabled sensors tend to be more expensive (due to the extra processing components and chips) though are becoming far more affordable and practical option as more and more sensors are brought out with this option.
SDI-12 communication may also have the limiting factor of taking around 20–30 seconds to take a measurement, however a resolution better than this is not a typical measurement interval in field applications, though it would be preferable to have this option and may very well be an option in the near future as electrical component technology increases.
Electrically the protocol is a three wire digital connection - data, ground and 12V. The data signal, using 5V logic levels, bears no resemblance to RS-485 or RS-232 although the timing is similar. The inline data is human readable as the data is transmitted in plain ASCII.
As specified by the SDI-12 Support Group, all SDI-12 communications are transmitted in ASCII
at 1200 baud
with 7 data bits and an even parity bit. A serial
break signal is sent by the master prior to any SDI-12 message to alert the connected slaves of the impending communication. Only the slave whose address matches the address in the sent message should respond. Checksums were introduced to the SDI-12 protocol with the V1.3 release.
ASCII
The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
, serial communications
Serial communications
In telecommunication and computer science, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are sent as a whole, on a link with several parallel channels...
protocol
Communications protocol
A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...
that was developed for intelligent sensory instruments that typically monitor environmental data
Environmental data
Environmental data is that which is based on the measurement of environmental pressures, the state of the environment and the impacts on ecosystems...
. These instruments are typically low-power (12 volt), are often used in remote locations, and usually communicate with a data logger
Data logger
A data logger is an electronic device that records data over time or in relation to location either with a built in instrument or sensor or via external instruments and sensors. Increasingly, but not entirely, they are based on a digital processor...
or other data acquisition
Data acquisition
Data acquisition is the process of sampling signals that measure real world physical conditions and converting the resulting samples into digital numeric values that can be manipulated by a computer. Data acquisition systems typically convert analog waveforms into digital values for processing...
device. In this master-slave
Master-slave
Master-slave may refer to:* Master-slave dialectic, philosophical concept* Master-slave * Master-Slave manipulator, see remote manipulator...
configuration, the data logger
Data logger
A data logger is an electronic device that records data over time or in relation to location either with a built in instrument or sensor or via external instruments and sensors. Increasingly, but not entirely, they are based on a digital processor...
or data acquisition
Data acquisition
Data acquisition is the process of sampling signals that measure real world physical conditions and converting the resulting samples into digital numeric values that can be manipulated by a computer. Data acquisition systems typically convert analog waveforms into digital values for processing...
device typically acts as the master (SDI-12 Recorder and Interogator) to the data monitoring instruments, which are the slaves (SDI-12 sensors). One master can communicate with multiple slaves, so the SDI-12 protocol requires that each device in the serial network be identified with a unique address, which is represented by a single ASCII
ASCII
The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
character.
This communication is achieved by digital communications along a single serial line.
The digital addressing system allows an SDI-Recorder to send out an address over a single line
that is occupied by up to 62 sensors with only the pre-configured sensor matching that address will respond (handshake), while the other sensors on the same line will not respond until called and typically stay in "sleep mode"(low power mode), until called (often in a sequence) at a later time by the SD-I Recorder (Master).
Advantages of this are the ability to use a single available data channel for many sensors (In many cases a technician may want to set up more sensors but is limited by the number of analogue channels that may be available on a particular Data Logger).
Another popular advantage is the ability to interface otherwise incompatible equipment.
This allows more sensors to be utilised on a limited number of channels, transmit over longer distances and save power.
SDI-12 enabled sensors tend to be more expensive (due to the extra processing components and chips) though are becoming far more affordable and practical option as more and more sensors are brought out with this option.
SDI-12 communication may also have the limiting factor of taking around 20–30 seconds to take a measurement, however a resolution better than this is not a typical measurement interval in field applications, though it would be preferable to have this option and may very well be an option in the near future as electrical component technology increases.
Electrically the protocol is a three wire digital connection - data, ground and 12V. The data signal, using 5V logic levels, bears no resemblance to RS-485 or RS-232 although the timing is similar. The inline data is human readable as the data is transmitted in plain ASCII.
As specified by the SDI-12 Support Group, all SDI-12 communications are transmitted in ASCII
ASCII
The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
at 1200 baud
Baud
In telecommunications and electronics, baud is synonymous to symbols per second or pulses per second. It is the unit of symbol rate, also known as baud rate or modulation rate; the number of distinct symbol changes made to the transmission medium per second in a digitally modulated signal or a...
with 7 data bits and an even parity bit. A serial
Serial communications
In telecommunication and computer science, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are sent as a whole, on a link with several parallel channels...
break signal is sent by the master prior to any SDI-12 message to alert the connected slaves of the impending communication. Only the slave whose address matches the address in the sent message should respond. Checksums were introduced to the SDI-12 protocol with the V1.3 release.
External links for documentation
- http://embedded-system-programming.googlegroups.com/web/sdi_documentation.pdf?gsc=AoB4IAsAAAA9i-tO7e2_GLNlOJ2brKeg