Data loggers utilize several types of digital communication protocols in order to support a wide range of sensors. Protocols can exist on both a hardware and/or software level. Some protocols define both the hardware and software (i.e. SDI-12), others only specify the hardware (i.e. RS-485), and still others only consist of the software definition (i.e. Modbus RTU).
When a protocol only defines the hardware such as RS-485, a software protocol such as Modbus RTU must also be selected to complete the communication interface. Modbus RTU over RS-485 is a very common industrial communication protocol that has been adapted for use in many applications including the environmental monitoring industry. However, Modbus RTU can also be implemented using RS-232 hardware in much the same manner.
All communication protocols are implemented differently. Some are full duplex (simultaneous transmit and receive is possible), others are half duplex (transmit and receive must take turns), some use a single wire (i.e. SDI-12), others use four (i.e. RS-422), some stream data constantly (i.e. NMEA), others respond to commands (i.e. SDI-12), some operate at 9600 baud data rates, others operate at 19,200 baud. They all have their advantages and disadvantages and the details regarding these are beyond the scope of this article. The important thing to remember is that in order to communicate data from one device to another, both must be using the same hardware and software protocol. This is why data loggers must support several different common communication protocols. Different sensors are built with different protocols (The most common in the environmental monitoring industry are RS-232, RS-485, and SDI-12), and the data logger must match these to read data from the sensor.
Another important factor to understand with regard to these protocols is the ability to connect multiple sensors to the same data bus. Some protocols (i.e. RS-485 Modbus RTU & SDI-12) allow for multiple sensors to be connected to the same data line. These sensors receive unique addresses (See Sensor Addresses) and can be accessed individually. Other protocols (NMEA & RS-232) only allow a single sensor on the data bus. NMEA is a software protocol and can be implemented on both RS-485 and RS-232, but regardless of the hardware utilized, only a single NMEA sensor can be connected to a data bus. This is true because NMEA is a streaming protocol that constantly outputs a data stream and if two of these were connected at the same time the data would collide and corrupt each other. On the other hand, the hardware protocol RS-232 is also limited to a single sensor due to how the signals are shared. If more than one sensor is connected, the transmit and receive voltages will interfere and corrupt the signal. Therefore, even if you implement the addressable Modbus RTU protocol using RS-232, only a single sensor can be connected at a time.
SDI-12 is a communication protocol that was specifically designed for the environmental monitoring industry. It has garnered wide acceptance and is used in a large number of sensors today. For a more detailed overview of SDI-12, please see our SDI-12 Protocol article.
As a final note, analog sensors are another common form of data transmission. While these are not a communication protocol in the same sense as the others discussed here, it is important to understand their existence in this discussion. Analog sensors rely on either a variable voltage or current to correspond to a particular parameter value. By reading the sensor’s output and correlating that with known scaling values or an equation, a data logger is able to calculate the corresponding parameter value. Analog sensors have become less common as the digital transition has continued, but they still exist in the industry.