USER MANUAL
TYPE ML-2012/13
Manufacturers of low power instruments
page 18
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 ML2011 is protected against transients
on the SDI-12 bus.
4.2.3 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:
SDI-12 is a half-duplex protocol, so the data-recorder has to switch between transmitting and receiving. A
convertor from RS232
SDI-12, must perform this task. However, it is not aware of the exact timing of
the protocol. Therefore it uses fixed (or configurable) delays to switch between Tx and Rx. After each byte
send by the convertor, it waits, during the fixed delay, for another character, and if it doesn’t arrive, it
switches to Rx. The intelligence needed to perform these tasks is mostly done by a microcontroller inside
the convertor, that’s the main reason for its high pricing. This method is doing the job for most cases, but
it is not as good as a real SDI-12 port. The real SDI-12 port is aware of the exact protocol-timings and
after the last character it switches to RX-mode immediately, without the delay. Therefore no replies are
missing. Your ML-2012/13 has a true SDI-12 port.
For more information on the SDI-12 protocol: see
www.sdi-12.org
4.3 RS232
Your ML-2012/13 comes with 2 RS232 ports, capable of baud rates of up to 115200 bps (230400 bps on
request). The ports are: port 1 and port 2. Port one is shared with the RS-485 port and port 2 is shared
with the SDI-12 port. The user can choose any combination he likes, as long as he is using only one at a
time. (e.g. you cannot share RS232 and RS485 one the same port). If a user needs RS232 and RS485
simultaneously, he needs to setup Port 2 also.
RS232 is a widely spread interface standard, which uses 3 wires (minimum) for data communication. It is
a so called asymmetric interface, that uses one wire for Tx, one for Rx and one for ground. It is called
asymmetrical, because it uses only one wire per signal. Therefore it’s susceptible for interference, and
hence, the maximum cable length is limited to 15meters.
Please keep in mind these limitation when you design your system.
RS232 is not a bus system, and therefore it is only allowed for one device to be connected to a RS232
port. So, the maximum number of serial
devices to connect to your ML-2012/13 is 2. RS232 sensors
should be connected to the data logger with their signals crossed. That is Rx
Tx.
We strongly recommend using RS485 instead of RS232, when cable length exceeds the 15m.
