MPS-2 Digital Sensor 1.02 Sensor Specifications
Contents
1. lstop bit Table 2 SDI 12 byte frame format MPS2 Digital Sensor 40854 40855 40856 1 Sensor Specifications Rev 1 02 break at least 12 milliseconds SDI 12 Data Line Decagon Devices Inc command response marking marking at least 8 33 milliseconds 8 33 milliseconds sensor must respond maximum time within 15 milliseconds maximum time 780 msec for aD command after a concurrent measurement for a D command after a concurrent measurement with CRC Figure 3 SDI 12 Timing 2365 NE Hopkins CT Pullman WA 99163 Page 9 of 9 509 332 27562. Specifications Water Potential Range 5kPa to 500 kPa pF 1 71 to pF 3 71 Resolution 0 1 kPa Accuracy 20 kPa from 5 to 100 kPa See user manual for more information regard higher kPa accuracies Temperature Range 40 C to 50 C Resolution 0 1 C Accuracy 1 C General Survival Environment 40 C to 60 C 0 to 100 RH Operating Temperature 0 C to 50 C no water potential measurement below 0 C Power requirements 3 6 15 VDC 0 03 mA quiescent 10 mA max during 150 ms measurement Dimensions 9 6 cm 1 x 3 5 cm w x 1 5 cm d Sensor diameter 3 2 cm Dielectric Measurement Frequency 70 MHz Measurement Time 150 ms milliseconds Output RS232 TTL with 3 6 volt levels or SDI 12 communication protocol Connector types 3 5 mm stereo plug or stripped amp tinned lead wires 3 Cable Length 5m standard custom cable length available upon request Input Output Circuitry A single line is used for both transmit and receive A 10 uH inductor and a 510 ohm resistor are in series with the RXD TXD line on the microprocessor with a 220 pF capacitor to ground The resistor and capacitor provide input protection The inductor minimizes RF interference See Figure 1 for an equivalent circuit diagram Decagon Devices Inc 2365 NE Hopkins CT Pullman WA 99163 509 332 2756 Page 2 of 9 MPS2 Digital Sensor 40854 40855 40856 1 Sensor Specifications Rev 1 02 220PF GND Figure 1 Equiva
3. for detailed information on the SDI 12 protocol Communication is at 1200 baud but logic levels are opposite to those for the initial transmission start bit high stop bit low to conform to SDI 12 protocol A communication starts with a SDI 12 break or any transmitted character This wakes the sensor No activity on the communication line for approximately 10 s returns it to sleep mode 30 uA current drain Since there is only one data line which is used for both transmit and receive the host must transmit its message and then quickly switch to receive mode to monitor the response All commands are of the form lt address gt lt command gt lt parameters gt The is always the termination character for every SDI 12 command Multiple device addresses are allowed so several probes can be connected to one port if they have different addresses and each can be addressed separately The can be used in place of an address in which case all devices connected to the port will respond SDI 12 Format Description The DO data string output by the sensor should be in a format similar to the one below lt addr gt 34 8 22 3 lt cr gt lt l1f gt Section Description lt addr gt Sensor SDI 12 Address A one character long SDI 12 address for the sensor It can be any alphanumeric value 0 9 a z A Z O is the default 34 8 Water potential in kPa Values range approximately from 7 kPa to 110 000 kPa Note in the case of
4. which is 0x30 or ASCII 0 TTL Format Description The data string output by the sensor should be in a format similar to the one below lt tab gt 34 8 22 3 lt cr gt yN lt cr gt lt 1f gt Section Description lt tab gt Tab character 0x09 signaling the start of the transmission 34 8 Water potential in kPa Values range approximately from 7 kPa to 110 000 kPa Note in the case of an error water potential will output an error value of 9999 The water potential calculation adds a small offset for real water potential values that calculate to 9999 so that the error value is a unique case 22 3 Temperature This number is the temperature of the sensor in degrees Celsius Note In the case of an error temperature will output an error value of Decagon Devices Inc 2365 NE Hopkins CT Pullman WA 99163 509 332 2756 Page 4 of 9 MPS2 Digital Sensor 40854 40855 40856 1 Sensor Specifications Rev 1 02 9999 lt cr 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 the MPS 2 sensor 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 tab gt 34 8 22 3 lt cr gt y See the fo
5. MPS2 Digital Sensor 40854 40855 40856 1 Sensor Specifications Rev 1 02 Unit Assembly s MPS 2 Digital Sensor 40854 40855 All Configurations 40856 Doc Document Type 1 02 Sensor Specifications Revisions REV By 1 Initial draft of MPS 2 specifications integrator s guide some revisions to output value ranges 3 Output waveform capture updated with logic analyzer Ooo S S E Decagon Devices Inc 2365 NE Hopkins CT Pullman WA 99163 509 332 2756 Page lof9 Il MPS2 Digital Sensor 40854 40855 40856 1 Sensor Specifications Rev 1 02 Description of Sensor and Measurements The MPS 2 digital sensor is designed to measure the water potential soil suction and temperature of soil and growing media Features The MPS 2 sensor measures soil water potential and soil temperature These fundamental measurements are needed for a variety of plant water flow and soil mechanical stress studies By combining these measurements into a single sensor it allows for more data logger ports to be used for additional environmental measurements Water potential is obtained by measuring the dielectric permittivity of a solid matrix porous ceramic disc through the utilization of capacitance frequency domain technology The MPS 2 incorporates high frequency oscillation allowing the sensor accurate measurements of water potential in any soil or soilless media with minimal salinity and textural effects III Measurement
6. an error water potential will output an error value of 9999 The water potential calculation adds a small offset for real water potential values that calculate to 9999 so that the error value is a unique case 2243 Temperature This number is the temperature of the sensor in degrees Celsius Note In the case of an error temperature will output an error value of 9999 lt cr gt lt lf gt This carriage return line feed combination signals the end of the measurement string Some Commonly Used SDI 12 Commands Command Description A lt new address gt change the sensor address an address can be an integer 0 9 or letter A Z a z I returns probe ID M measure probe values DO return measured values Decagon Devices Inc 2365 NE Hopkins CT Pullman WA 99163 509 332 2756 Page 6 of 9 MPS2 Digital Sensor 40854 40855 40856 1 Sensor Specifications Rev 1 02 SDI 12 Examples The characters in bold are what was sent to the sensor 1I 113DECAGON MPS2 108 Identify sensor with address of 1 Return Sensor Address 1 SDI12 Version 13 COMPANY DECAGON SENSOR TYPE MPS2 SENSOR VERSION 108 1M 10012 Measure parameters of sensor with address of 1 Return sensor address is 1 data are ready in 001 ms 2 values will be sent 1D0 1 82 6 21 8 Send the values Return sensor address is 1 82 6 kPa for water potential 21 8 C for temperature 210 This returns the sensor ad
7. dress Return sensor address is 0 OAL 1 Change the sensor address to 1 Starting address is 0 Return new sensor address is Decagon Devices Inc 2365 NE Hopkins CT Pullman WA 99163 509 332 2756 Page 7 of 9 MPS2 Digital Sensor 40854 40855 40856 1 Sensor Specifications Rev 1 02 SDI 12 Compliance Decagon s SDI 12 sensors recognize all SDI 12 commands outlined in Version 1 3 of the SDI 12 Protocol see http www sdi 12 org for more detailed information Here is a list of differences between the SDI 12 standard and Decagon s SDI 12 sensors e Upon excitation a TTL string is printed prior to enabling SDI 12 takes around 75ms Parity Bits are ignored R amp RC Commands return a reading after about 50ms Timeout Power Down is around 5 seconds instead of 100ms After Timeout Power Down sensors may respond to a break that s less than 6 5 ms SDI 12 Diagrams SDI 12 Bus serial data line SDI 12 data recorder Figure 1 The SDI 12 Bus All sensors must have a unique address At most 62 sensors can be connected to a single bus Condition Binary state Voltage range marking 1 0 5 to 1 0 volts spacing 0 3 5 to 5 5 volts transition undefined 1 0 to 3 5 volts Table 1 Logic and voltage levels for serial data Decagon Devices Inc 2365 NE Hopkins CT Pullman WA 99163 509 332 2756 Page 8 of 9 1 start bit 7 data bits least significant bit transmitted first 1 parity bit even parity
8. it The voltage levels are 0 3 6 V and the logic levels are TTL active low Each data byte consists of a start bit low 8 data bits least significant bit first and a stop bit high see Figure 3 and 4 for a serial stream example Decagon Devices Inc 2365 NE Hopkins CT Pullman WA 99163 509 332 2756 Page 3 of 9 MPS2 Digital Sensor 40854 40855 40856 1 Sensor Specifications Rev 1 02 10 ms 20 ms 30 ms 40 ms 50 ms 60 ms 70ms 8Qms 90ms 10 ms 20 ms 30 ms 40 ms 50 ms 60 ms 70 ms 80 ms 90ms 10 ms 20 ms 3q Figure 3 Excitation and serial output stream Output levels are not absolute Both excitation and output are zero or I V P TO HOTTA OCA TMC A initially Approximately 40 ms after the excitation goes high the serial out line goes high It stays high for 15 ms and then goes low for the start bit of the serial stream Twelve characters are shown 100 100 100 r When the carriage return character is received the excitation is shut off a a ee ee ee Amm A Aa esan Util ity a ee I A S pa Ht E o pS S 80 Time ms Figure 3 Expansion of first few bits of serial stream from Fig 1 showing the start and stop bits and the data bits Baud rate is 1200 so one byte 8 data bits plus start and stop takes 0 833 ms Bytes are transmitted Isb least significant bit first so the first character 10001100 represents 00110001 or 0x31 which is ASCII 1 The next character is 00001100
9. lent Circuit Diagram If using multiple sensors on a bus you may use the equivalent circuit to simulate the load of additional sensors up to 62 on a single SDI 12 line Sensor Excitation The sensor connects through a 3 wire cable with a stereo connector or bare wire interface The three connections are Excitation Ground and Serial Out Data The connector tip white wire is Excitation the connector base shield is ground and the intermediate ring red wire is Serial Out see Figure 2 below for stereo jack signal diagram Ground bare Serial Out red Power white Figure 2 Stereo Jack Signal Diagram The signals from base to tip ground serial out and power The excitation is 3 6 to 15 volts Current drain during the water potential measurement approximately 10 ms in duration can be as high as 10 mA After data transmission the sensor will pull around 3mA for about 6 seconds It will then go into a sleep state and draw around 30uA until a SDI 12 break command is received Be sure that the selected power supply can provide this current without being pulled below the 3 6 V level If all sensors are excited at the same time all of them will draw 10mA for 10ms The system will need to source enough current to supply the number of sensors in the system up to 62 x times 10mA Total current x 10mA TTL Communication The serial out uses 1200 baud asynchronous CMOS with 8 data bits no parity and one stop b
10. llowing function for an example of how to implement the checksum algorithm in C lt cr gt lt lf gt The carriage return OxOd and line feed Ox0a are used to signal the end of the meta data section and the end of the transmission 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 tab gt 34 8 22 3 lt cr gt y and the returning value would be the character N char CalculateChecksum char Response int length sum 0 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 Q crc sum 64 32 return crc TTL Typical Outputs Typical output while the sensor is suspended in air 20 C will be approximately 100000 kPa and 20 0 C If the sensor is fully saturated in 20C DI water the outputs will be approximately 8 kPa and 20 0 C SDI 12 Communication When power is applied to the probe it transmits the TTL data values as previously discussed It then enters a low power state waiting for input The commands it Decagon Devices Inc 2365 NE Hopkins CT Pullman WA 99163 509 332 2756 Page 5 of 9 MPS2 Digital Sensor 40854 40855 40856 1 Sensor Specifications Rev 1 02 recognizes follow the SDI 12 protocol see http www sdi 12 org
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