Overview of Sensor Communication Protocols
Data loggers utilize several digital communication protocols to support various sensors. Protocols can exist on both a hardware and 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 prevalent industrial communication protocol adapted for many applications, including environmental monitoring. However, RS-232 hardware also supports the Modbus RTU protocol.
All communication protocols differ in their wiring and communication. Some protocols 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), and others use four (i.e., RS-422). Some protocols stream data constantly (i.e., NMEA), and others respond to commands (i.e., SDI-12). Protocols can operate over a wide range of baud rates to control the speed of information transfer (e.g., 1200 – 115200 Bd).
Different sensors are built with various protocols, the most common being RS-232, RS-485, and SDI-12. Each protocol has advantages and disadvantages and details beyond this article’s scope. The critical aspect is that to communicate data from one device to another, both must use the same hardware and software protocol and communicate at the same baud rate. Thus, data loggers must be built to communicate in multiple protocols.
Another essential factor concerning 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 multiple sensors to be connected to the same data line. These sensors receive unique 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 that can be implemented on RS-485 and RS-232; however, regardless of the hardware, only a single NMEA sensor can be connected to a data bus.
NMEA is a streaming protocol that constantly outputs a data stream, and if two sensors were connected simultaneously, the data would collide and corrupt each other. Since signals are shared, the hardware protocol RS-232 is also limited to a single sensor. 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 simultaneously.