ML-x17 DATA LOGGER MANUAL
Manufacturers of low power instruments
page 94
4.32 SDI-12
The ML-x17DS or ML-x17ADS is provided with and SDI-12 port, but it is shared with the RS-232 and RS-
485 port. You can add an (additional) SDI-12 port to any ML-x17 by means of an ML-OI-COM-SDI12 option
board.
When SDI-12 is selected, it acts like a SDI-12 recorder and its specific SDI-12 commands are embedded
in the driver of the input-sensor. So, the user can easily select his sensor and specify its SDI-12-adress.
For more information see the description of your SDI-12 sensor.
4.32.1 SDI-12 Hardware
The SDI-12 standard is a very commonly used interface-standard. The signal levels are quite different from
those of RS232 and RS485. So, you cannot connect a SDI-
12 sensor to a RS232 or RS485 port, it won’t
work. The use of convertors between RS232/485 and SDI-12 is discouraged, because of the high pricing
of the convertors and the bad performance (see note).
4.32.2 SDI-12 Wiring
The SDI-12 electrical interface uses the SDI-12 bus to transmit serial data between SDI-12 data recorders
and sensors. The SDI-12 bus is the cable that connects multiple SDI-12 devices. This is a cable with three
conductors:
1) a serial data line, 2) a ground line and 3) a 12VDC line
The wiring length between a sensor and the data-recorder must not exceed 60 meters. The maximum
number of sensor connected to a SDI-12 bus is limited to 10. The data logger is protected against transients
on the SDI-12 bus.
4.32.3 SDI-12 Baud Rate and Frame Format
The baud rate for SDI-12 is 1200. Frame format is as follows:
1 start bit
7 data bits, least significant bit transmitted first
1 parity bit, even parity
1 stop bit
Note:
