Matric Potential Sensor 2 & 6
Contents
1. DDI SERIAL TIMING DDI serial communication sends characters at 1200 bits per second Each character has 1 start bit 8 data bits LSB first no parity bit and 1 stop bit Active high or non inverted logic levels sTART DO D1 D2 D3 D4 D5 D6 D7 stop Example serial transmission of the character 9 Ox39 Upon power up the sensor will pull the data line high within 100 mS to indicate that the sensor is taking a reading When the reading is complete the sensor begins sending the serial signal out the data line Once the data is transmitted the sensor goes into SDI 12 communication mode In order to get another serial signal the sensor must be power cycled POWER Measurement Signal at most 100ms DDI Response SDI 12 Mode t DATA x lt gt DECAGON For support contact us via email at support decagon com 11 DEVICES or call us at 1 509 332 5600 between 7AM to 5PM PST MPS 2 and MPS 6 Sensor Integrator Guide R2 Note Sometimes the signaling from the sensor can confuse typical microprocessor UARTs The sensor holds the data line low while it takes measurements It raises the line high to signal the logger that it will send a measurement as documented above Then the sensor may take some additional measurements before starting to clock out the first data byte starting with a typical start bit low Once the first start bit is sent typical serial timing is valid however the signal transitions before this point are n2. DECAGON S SDI 12 IMPLEMENTATION Decagon s sensors use a low impedance variant of the SDI 12 standard sensor circuit see the Equivalent Circuit Diagram This allows for up to 62 sensors to be connected onto the bus at one time instead of the 10 that is stated in the standard Keep in mind that the more sensors you have on a bus the more difficult it will be to isolate and remove the faulty sensor and restore communication to the remaining sensors on your SDI 12 bus Out of the factory all Decagon sensors start with SDI 12 address zero and print out the DDI Serial string during the power up time specified in the Electrical and Timing Characteristics section See the start up sequence diagram in the DDI Serial section for a detailed diagram This can be interpreted by non Decagon SDI 12 sensors as a pseudo break condition followed by a random series of bits To circumvent this issue sensors with firmware version 1 35 and newer omit the DDI Serial string when their SDI 12 address is non zero Simply changing the sensor addresses on the SDI 12 bus to non zero values will prevent any race conditions on the data line during sensor power up After the power up time our sensors are fully compatible with all the commands listed in the SDI 12 Specification v1 3 except for the continuous measurement commands aRO aR9 and aRCO aRC9Q SENSOR BUS CONSIDERATIONS An SDI 12 bus isn t something you can set up and just walk away Sensor buses requi
3. must use the concurrent command See the SDI 12 Specifications v1 3 document for more information DECAGON For support contact us via email at support decagon com g DEVICES or call us at 1 509 332 5600 between 7AM to 5PM PST MPS 2 and MPS 6 Sensor Integrator Guide R2 DATA COMMAND aDO The data command is used to retrieve the measurement response from the sensor Each value is delimited by the sign of the value that can be either positive or negative If the sensor is reading incorrectly the sensor will output 9999 for the erroneous value Here is an example output from one of our sensors the logger command is in bold 1D0 1 34 8 22 3 PARAMETER FIXED LENGTH DESCRIPTION 1D0 4 Chars Request from the data logger asking the sensor with address 1 to return the response from a measurement A measurement command must be sent prior to sending the data command 1 1 Char Sensor Address Pre pended on all responses this allows you to know which sensor on your bus is returning the following information ao g Variable Water potential in kPa Values range from approximately 10 kPa to 110000 kPa 22 3 Variable Temperature measurement in C t The measurement and corresponding data command are intended to be used back to back If you would like to start a measurement on more than one sensor at a time you must use the concurrent command See the SDI 12 Specifications document for more information CHANGE
4. 00 between 7AM to 5PM PST MPS 2 and MPS 6 Sensor Integrator Guide R2 DDI SERIAL CHECKSUM Here is an example of how to calculate the checksum crc in C In this case the string passed to the function would be lt 09 gt 34 8 22 3 lt 0D gt y and the returning value would be the character N char CalculateChecksum char Response int length sum i crc Finding the length of the response string length strlen Response Adding characters in the response together for i 0 i lt length i sum Response i Converting checksum to a printable character crc sum 64 32 return crc DECAGON For support contact us via email at support decagon com DEVICES or call us at 1 509 332 5600 between 7AM to 5PM PST 13
5. 9 6 cm X 3 5 cm X 1 5 cm CABLE LENGTH 5 meters Custom cable lengths are available for an additional cost ELECTRICAL AND TIMING CHARACTERISTICS PARAMETER MIN TYP MAX UNITS Supply Voltage VCC to GND 3 6 15 y Digital Input Voltage logic high 2 8 3 3 9 V Digital Input Voltage logic low 0 3 0 0 8 V Power Line Slew Rate 1 V mS Current Drain during measurement 3 0 3 0 10 0 mA Current Drain while asleep 0 03 mA Operating Temperature Range 40 50 C Power Up Time DDI Serial 100 mS Power Up Time SDI 12 100 150 200 mS Measurement duration 150 200 mS Cable Capacitance meter 250 pF Cable Resistance meter 35 mQ EQUIVALENT CIRCUIT DIAGRAM CONNECTION TYPES DECAGON For support contact us via email at support decagon com 2 DEVICES or call us at 1 509 332 5600 between 7AM to 5PM PST MPS 2 and MPS 6 Sensor Integrator Guide R2 PIGTAIL CABLE Ground roam g Data E ame STEREO CABLE Pies al Y Power GND NOTE This is a low impedance variant of the recommended SDI 12 Specification This allows up to 62 sensors to be maintained on a bus 220PF SAFETY PRECAUTIONS Our probes are built to the highest standards but misuse or improper protection or improper installation may break your sensor and possibly void the manufacturer s warranty Before integrating your sensors into your sensor network make sure you have followed the recomme
6. ADDRESS COMMAND aAb The change address command is used to change a sensor s address to a new address All other commands support the wild card character as the target sensor address except for this command All of Decagon s sensors have a default address of 0 zero out of the factory In order to use more than one of our sensors on a bus system you must change the addresses so that they are all unique Supported addresses are alpha numeric i e a z A Z and O 9 Changing address while connected to a bus is not recommended Here is an example output from one of our sensors the logger command is in bold 1A0 0 PARAMETER FIXED LENGTH DESCRIPTION 1A0 4 Chars Request from the data logger asking the sensor to change the address of sensor with address 1 to a new address of 0 zero 0 1 Char New Sensor Address For all subsequent commands this new address will be used for the target sensor DECAGON For support contact us via email at support decagon com 9 DEVICES or call us at 1 509 332 5600 between 7AM to 5PM PST MPS 2 and MPS 6 Sensor Integrator Guide R2 ADDRESS QUERY COMMAND While disconnected from a bus the address query command can be used to determine which sensor you are currently communicating with Sending this command over a bus will cause a bus contention where all the sensors will respond simultaneously and corrupt the data line This command is helpful if you are trying to isolate a fa
7. DECAGON DEVICES 2365 NE Hopkins Ct Pullman WA 99163 USA APPLICATIONS Deficit irrigation monitoring and control Water potential monitoring in the vadose zone Crop stress Waste water drainage studies Plant water availability SDI 12 implementation ADVANTAGES Digital sensor communicates two measurements over a serial interface Robust thermistor for accurate temperature measurements Low input voltage requirements Low power design supports battery operated data loggers Robust epoxy encapsulation to resist corrosive environments Supports SDI 12 or DDI Serial 1 wire serial communications protocols Modern design optimized for low cost sensing Does not require a skilled operator Can measure drier systems where tensiometer cavitation is a concern Needs no user maintenance Matric Potential Sensor 2 amp 6 Water Potential and Temperature DESCRIPTION The Decagon MPS 2 and MPS 6 sensors measure a wide range of soil water potentials without user maintenance and factory calibration Unlike tensiometers which need a skilled operator this dielectric water potential sensor needs no maintenance It can be packed into a hole plugged into a data logger and left to log water potential data While the MPS sensors don t have the accuracy of UMS tensiometers its extended range makes this sensor ideal for measuring the water potential in natural systems or other drier syst
8. age driving over sensor cables tripping over the cable not leaving enough cable slack during installation or poor sensor wiring connections are just some of many factors that can cause cabling issues DECAGON For support contact us via email at support decagon com 4 DEVICES or call us at 1 509 332 5600 between 7AM to 5PM PST MPS 2 and MPS 6 Sensor Integrator Guide R2 SENSOR COMMUNICATION TYPES Decagon digital sensors feature a 1 wire serial interface for communicating sensor measurements The sensor supports two different protocols SDI 12 and DDI Serial Both interfaces are documented in this guide Each protocol has advantages and implementation challenges There are differences in voltage levels logic levels and signal timing for each protocol Please contact Decagon with your specific requirements if the protocol choice for your application is not obvious SDI 12 INTRODUCTION SDI 12 is a standards based protocol for interfacing sensors to data loggers and data acquisition equipment Multiple sensors with unique addresses can share a common 3 wire bus power ground and data Two way communication between the sensor and logger are possible by sharing the data line for transmit and receive as defined by the standard Sensor measurements are triggered by protocol command DDI SERIAL INTRODUCTION The DDI Serial protocol is the method used by the Decagon s family of data loggers for collecting data from the sensor This protoco
9. ems where cavitation of tensiometers is a concern The added temperature measurements can be used to determine approximate soil water potential in frozen soils AUDIENCE Decagon provides the information in this integrators guide to help MPS customers establish comm unication between these sensors and their data acquisition equipment or field data loggers Customers using data loggers that support SDI 12 sensor communications should consult the user s manual for their data logger These sensors are fully integrated into Decagon s system of plug and play sensors cellular enabled data loggers and data analysis software MEASUREMENT SPECIFICATIONS MPS 2 MPS 6 Water Potential Temperature Water Potential Temperature Accuracy 25 of reading 2kPa 1 C 10 of reading 2kPa 1 C from 9 kPa to 100 kPa from 9 kPa to 100 kPa Resolution 0 1 kPa 0 1 C 0 1 kPa 0 1 C Range 9 kPa to 100000 kPa pF 40 C to 50 C 9 kPa to 100000 kPa pF 40 C to 50 C 2 00 to pF 9 01 2 00 to pF 9 01 See user manual for more information regarding accuracy at lower water potentials DECAGON DEVICES For support contact us via email at support decagon com or call us at 1 509 332 5600 between 7AM to 5PM PST MPS 2 and MPS 6 Sensor Integrator Guide R2 PHYSICAL CHARACTERISTICS SENSOR IMAGE SENSOR NAME Matric Potential Sensor 2 or Matric Potential Sensor 6 DIMENSIONS
10. iled sensor Here is an example output from one of our sensors the logger command is in bold 210 PARAMETER FIXED LENGTH DESCRIPTION 2 2 Chars Request from the data logger asking for a response from any sensor listening on the data line 0 1 Char Sensor Address Returns the sensor address to the currently connected sensor DECAGON For support contact us via email at support decagon com 10 DEVICES or call us at 1 509 332 5600 between 7AM to 5PM PST MPS 2 and MPS 6 Sensor Integrator Guide R2 DDI SERIAL COMMUNICATION The DDI Serial communications protocol is ideal for systems that have dedicated serial signaling lines for each sensor or use a multiplexer to handle multiple sensors The serial communications are compatible with many TTL serial implementations that support active high logic levels using 0 3 6 volts signal levels When the sensor is first powered it automatically makes measurements of the integrated transducers then outputs a response over the data line Systems using this protocol control the sensor excitation to initiate data transfers from the sensor This protocol is subject to change as Decagon improves and expands our line of digital sensors and data loggers NOTE Out of the factory all Decagon sensors start with SDI 12 address zero and print out the startup string when power cycled On MPS 2 sensors with firmware version 1 35 and newer the start up string is omitted when the address is non zero
11. l uses the data line configured to transmit data from the sensor to the receiver only simplex Typically the receive side is a microprocessor UART or a general purpose IO pin using a bit bang method to receive data Sensor measurements are triggered by applying power to the sensor INTERFACING THE SENSOR TO A PC The serial signals and protocols supported by the sensor require some type of interface hardware to be compatible with the serial port found on most personal computers or USB to Serial adapters There are several SDI 12 interface adapters available in the marketplace however Decagon has not tested any of these interfaces and cannot make a recommendation of which adapters work with Decagon sensors Decagon s Em50 data logger and ProCheck hand held both are able to operate as a computer to sensor interface for making on demand sensor measurements DECAGON For support contact us via email at support decagon com 5 DEVICES or call us at 1 509 332 5600 between 7AM to 5PM PST MPS 2 and MPS 6 Sensor Integrator Guide R2 SDI 12 COMMUNICATION SDI 12 is a common bus communication protocol that allows you to connect multiple sensors to a common data line The SDI 12 protocol requires a unique alpha numeric sensor address for each sensor on the bus so that a data logger can send and receive readings to specific sensors Visit www sdi 12 org to download the latest SDI 12 Specification or find out more information about the standard
12. nded installation instructions and have the proper protections in place to safeguard your sensor from damaging interference LIGHTNING AND SURGE PROTECTION Probes have built in circuitry that protects them against common surge conditions Installations in lightning prone areas however require special precautions especially when probes are connected to a well grounded third party logger Visit our website and read the following application note for more information Lightning Surge Suppression and Standard Grounding Practices POWER AND GROUNDING Sensor versions 1 27 and older start measurements as soon as they are powered For older sensors ensure that there is sufficient power to support the maximum sensor current drain for all the sensors on the bus simultaneously Our sensor protection circuitry may be insufficient if your data logger is improperly powered or grounded Refer to your data logger s installation instructions Improper grounding may affect the sensor output as well as the sensor performance For Decagon loggers visit our website and read the following article for more information Lightning Surge Suppression and Standard Grounding Practices DECAGON For support contact us via email at support decagon com 3 DEVICES or call us at 1 509 332 5600 between 7AM to 5PM PST MPS 2 and MPS 6 Sensor Integrator Guide R2 CABLE PROBLEMS Cable problems can lead to severed cables or disconnected sensors Rodent dam
13. ommand Response 1o 1 1 1 f i Marking Marking i Sensor must respond Maximum time within 15 ms MAXIMUM TIME 380 mS Most Commands 780 mS For a D command after a concurrent measurement 810 mS For a D command after a concurrent measurement with CRC DECAGON For support contact us via email at support decagon com DEVICES or call us at 1 509 332 5600 between 7AM to 5PM PST MPS 2 and MPS 6 Sensor Integrator Guide R2 COMMON SDI 12 COMMANDS Below is a list of common SDI 12 commands that are often used in an SDI 12 system and the corresponding responses from our sensors INFO COMMAND aI The information command can be used to obtain a variety of detailed information about the sensor you are connected to Here is an example output from one of our sensors the logger command is in bold 1I 113DECAGON MPS 2 135631800001 PARAMETER FIXED LENGTH DESCRIPTION 11I 3 Chars Request from the data logger asking for information from sensor address 1 1 1 Char Sensor Address Pre pended on all responses this allows you to know which sensor on your bus is returning the following information T3 2 Chars Indicates that the target sensor supports SDI 12 Specification v1 3 2 characters DECAGON 8 Chars The Vendor Identification String The parameter is the same for all our Decagon sensors MPS 2 6 Chars The Sensor Model String This string is specific to the sensor type For the Matric Potential Senso
14. ot serial signaling and may be misinterpreted by the UART DDI SERIAL RESPONSE The data string output by the sensor should be in a format similar to the one below lt 09 gt 34 8 22 3 lt 0D gt yN lt 0D gt lt 0A gt Section Description PARAMETER DESCRIPTION lt 09 gt Tab character signaling the start of the data transmission 34 8 Water potential in kPa Values will typically range from 10 kPa to 110000 kPa 22 3 Temperature This number is the temperature of the sensor in degrees Celsius In this example 22 3 indicates that the sensor is measuring a temperature of 22 3 C lt 0D gt This carriage return character signals the end of the measurement string and start of the meta data string Everything below is this line is meta data y Sensor Type This character is used to indicate the sensor type y is used for MPS 2 sensors lowercase L is used for the MPS 6 N Checksum This one character checksum is used in our instruments to ensure that the data transmitted are valid The checksum is used for sections listed above lt 09 gt 34 8 22 3 lt 0DSy See the following function for an example of how to implement the checksum algorithm in C lt 0D gt lt 0A gt The carriage return and line feed are used to signal the end of the meta data section and the end of the transmission DECAGON For support contact us via email at support decagon com 12 DEVICES or call us at 1 509 332 56
15. r 2 the string is MPS 2 For the Matric Potential Sensor 6 the string is MPS 6 351 3 Chars The Sensor Version This number is divided by 100 to give our sensor version In this case the Decagon sensor version is 3 51 631800001 Up to 13 Chars The Sensor Serial Number The is a variable length field For older sensors this may be omitted Newer sensors may contain serial number information here In the event that the fixed length is longer than the parameter the trailing characters will be populated with space characters MEASUREMENT COMMAND aM The measurement command can be used to begin a sensor measurement Here is an example output from one of our sensors the logger command is in bold 1M 10012 PARAMETER FIXED LENGTH DESCRIPTION 1M 3 Chars Request from the data logger asking the sensor with address 1 to begin a measurement l 1 Char Sensor Address Pre pended on all responses this allows you to know which sensor on your bus is returning the following information 001 3 Chars The measurement data will be available after this specified time in seconds If a sensor finishes early it will initiate a service request SDI 12 address and a return character line feed 2 1 Char Number of values that will be returned t The measurement and corresponding data command are intended to be used back to back If you would like to start a measurement on more than one sensor at a time you
16. re regular checking sensor upkeep and sensor trouble shooting If one sensor goes down that may take down the whole bus even if the remaining sensors are functioning normally Power cycling the SDI 12 bus when a sensor is failing is okay but we do not recommend scheduling power cycling events on an SDI 12 bus more than once or twice a day Many factors influence the effectiveness of your bus configuration and many questions should be answered How often should data be checked How is the data delivered to the end user How do isolate a faulty sensor Visit our website and read our application note and view our virtual seminars on Best Practices for SDI 12 Sensor Networks DECAGON For support contact us via email at support decagon com 6 DEVICES or call us at 1 509 332 5600 between 7AM to 5PM PST MPS 2 and MPS 6 Sensor Integrator Guide R2 SDI 12 TIMING SDI 12 communication sends characters at 1200 bits per second Each character has 1 start bit 7 data bits LSB first 1 even parity bit and 1 stop bit Active low or inverted logic levels start DO D1 D2 D3 D4 D5 D6 EP stop Example SDI 12 transmission of the character 1 0x31 All SDI 12 commands and response must adhere to the following format on the data line Both the command and response are preceded by an address and terminated by a carriage return line feed combination at least 8 33 at least 8 33 DATA LOGGER SENSOR Break at least 12ms C
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