Decagon Devices GS3 Handbuch - Seite 5

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GS3 Integrator Guide

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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.

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, GS3 sensors with firmware version 3.37 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 (aR0 – aR9 and aRC0 – aRC9).

SENSOR BUS CONSIDERATIONS

An SDI-12 bus isn't something you can set up and just walk away. Sensor buses require 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 I 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.

For support contact us via email at

or call us at 1.509.332.5600 between 7AM to 5PM PST