How to Build a SDI-12 Bus
Introduction:
Although the use of SDI-12 allows one to plug up to 62 sensors into one data logger
port, these ports are only large enough to allow about four sensors to be plugged in
at a time. The use of a bus allows up to 62 sensors in one port. A bus uses three
separate terminal strips that you can wire each individual sensor into and then use
one outgoing wire to connect to your data logger. Busses are easy to make and can
save a considerable amount of time and effort.
A completed SDI-12 bus.
1. Equipment Needed:
1.1 Base for the bus (see explanation below)
1.2 3 terminal strips (see explanation below)
1.3 Three-wire cable (see explanation below)
1.4 Wire strippers
1.5 Soldering iron
1.6 Solder
1.7 Flux
1.8 Multimeter (see explanation below)
1.9 Optional Stereo plug adaptors (see explanation below)
1.10 Optional Accessible waterproof casing (see explanation below)
1.1.1 Perforated prototyping board is ideal because it has small holes that
work extremely well for sticking the pins of the terminal strips through,
however busses can be made out of a wide variety of materials, such as
cardboard, plastic, or anything else that will be durable enough for the
environment in which the bus is to be used. Another simpler option is to tape
the terminal strips together with electrical tape.
1.2.1 You will need three terminal strips because SDI-12 is a three wire
setup. These can be any sort of terminal strip that can secure at least one
wire in each port and have pins on the bottom to allow a wire to be soldered
to the bottom. Each port will be used for a specific wire (communication,
excitation, and ground). If terminal strips that are long enough cannot be
found, multiple terminal strips may be lined up end to end to allow more
sensors to be plugged in to the bus.
A terminal strip with pins along the bottom.
1.3.1 A three-wire cable (communication, excitation, and ground). This will
be used to connect each of the ports on the terminal strips to one port on a
data logger. The cable should be at least four times as long as the terminal
strips.
1.8.1 A multimeter to test each port and make sure that it is communicating
with the main cable.
1.9.1 Optional These are only necessary if the sensors that are to be used do
not have a pigtail end that can plug in directly to the terminal ports. They are
also more convenient because one can simply plug the sensors in instead of
having to wire each of the three wires for each sensor into each individual
port.
Stereo plug adaptors.
1.10.1 Optional If the bus is to be placed outside, some sort of accessible
waterproof casing must be used for protection. Make sure that you will be
able to access the bus for troubleshooting in case something goes wrong.
2. Building a Bus:
2.1 Make sure that there are holes in the base that will allow the pins
in your terminal strips to come through far enough to allow ease of
soldering. This is especially necessary if perforated prototyping board
is to be used. If cardboard is to be used, the pins of the terminal strips
may simply be punched through. If plastic or other materials are to be
used, holes must be drilled to allow the pins to fit through; and if the
terminal strips are simply to be taped together with electrical tape,
this step can be ignored.
2.2 The base must be cut to the appropriate size for the terminal
strips. With perforated prototyping board, score the board with an
exacto knife and use pliers to break off the excess.
2.3 Stick the terminal strip pins through the holes in the base, making
sure that the wires will not touch when soldered, also making sure
that that arrangement will be possible to solder.
Terminal strip pins stuck through perforated prototyping board.
2.4 Using wire strippers, strip the cable to about an inch longer than
the terminal ports. Then strip each of the three wires to about a half
inch longer than the length of the terminal ports. Twist the stranded
wire so that it doesn’t separate. The wires must then be tinned. This
can be done in either of two ways.
The correct length of wire compared to the terminal strip.
2.4.1 Tin each of the three wires by dipping the end in flux and then
dipping the end in a pot of solder.
2.4.2 Tin each of the three wires by dipping the end in flux, then
heating the end with a soldering iron. Once end is sufficiently heated,
touch solder to iron and let solder wick into the strands of the wire.
There should only be enough solder on the wire to fill the gaps
between the strands.
The correct way to tin using a soldering iron. There should only be
enough solder used as to completely fill the gaps between the strands of
the wire.
2.5 Line up the first wire next to the first set of terminal strip pins.
Solder by first heating up the pin and wire with a soldering iron, and
then applying solder. Remember that a good solder joint should be
concave, not convex in shape. This means that there should not be a
large pile of solder on top of the pin. Repeat this soldering process for
each pin on the first terminal strip. Then repeat this step for each of
the other two terminal strips.
The correct solder joints have only the bare minimum amount of solder
to connect the wire to the prongs, while on the incorrect joint there is
much excess solder.
2.6 To test the terminal strips, take a multimeter and set it to read
resistance (ohms). Touch one of the ends of the multimeter cable to
the first port on the terminal strip. If there is no break in
communication, the display should read close to zero. If there is, the
display will read in the millions or be unable to obtain a value. If this
should happen, the prong that corresponds to that port may have to
be soldered again. Repeat this step on all other ports. This step is
absolutely essential because if one of the ports on the bus is not
communicating, the bus may not function correctly.
Correct placement of multimeter on port.
2.7 Optional If stereo plug adaptors are to be used, insert them into
the terminal strips.
2.8 The bus is now finished and ready to connect to the sensors. Make
sure that the bus is both accessible and weatherproof, if it is to be
placed outside now would be the time to enclose it in its weatherproof
casing. Make sure to check data often, because if one sensor breaks,
the entire network will go down along with it until the sensor is
replaced. For this reason the casing will need to be accessible for
testing and replacement as needed.