39
13 Supplement
VEGAPULS C 21 • SDI-12
58342-EN-200121
Protection class
III
Pollution degree
4
13.2 Basics SDI-12
Bus description
SDI-12 (1200 bits/sec serial digital interface) is an asynchronous serial communication protocol
for intelligent environmental data acquisition sensors. These sensors typically have a low voltage
requirement (12 V DC), are used at remote measuring points and communicate with data loggers or
other measurement data acquisition systems. The protocol takes into account a master/slave con-
figuration where the master (SDI-12 data logger) requests measurement data from the intelligent
sensors (SDI-12 sensors) with a defined address. The electrical connection is made via a three-core
cable for supply and data transmission. The data are transmitted asynchronously as ASCII charac-
ters as 5 V levels similar to RS 232.
Bus architecture and communication process
At least 10 participants can be connected to an SDI-12 bus with a respective cable length of 60 m.
Longer cable lengths are also possible with fewer bus participants. The SDI-12 standard allows a
sleep mode at the sensor with minimal power consumption, which is interrupted by a signal from
the master. To identify the sensor, each command from the master contains the address of the
addressed sensor as the first character. Sensors that are not addressed ignore the command and
return to sleep mode.
1
2
Data
Data
Data
Data
Fig. 23: Bus architecture SDI-12
1 SD-12-Master
2 Bus participant
Basic Commands
Command
Response
Description
Break
-
A data recorder starts a request by transmitting a
break
a!
a<CR><LF>
Acknowledge Active
aI!
aiiccccccccmmmmmmvvvxx..
xx<CR><LF>
Send Identification
aAb!
b<CR><LF>
Change Adress
