SDI-12 allows anyone to plug up to 62 wires into one data logger port, but the common guillotine port is only large enough to allow about four sensors to be jammed in at once. This is where the bus comes in. SDI-12 buses are surprisingly easy to make and can be attempted by any person with some soldering skills. Below are step-by-step instructions on how to build your own bus. To see these instructions in PDF form, click the link below.
1. Equipment Needed:
1.1 Base for the bus (see explanation 1.1.1 below)
1.2 3 terminal strips (see explanation 1.2.1 below)
1.3 Three-wire cable (see explanation 1.3.1 below)
1.4 Wire strippers
1.5 Soldering iron
1.8 Multimeter (see explanation 1.8.1 below)
1.9 Optional Stereo plug adaptors (see explanation 1.9.1 below)
1.10 Optional Accessible waterproof casing (see explanation 1.10.1 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 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.
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.
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.
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.
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.
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.
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.
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.