There are several communications protocols that allow multiple sensors to be connected to the same data line. When this is done, the sensors are given an address in order to allow the data logger to identify the device to which it is communicating. The two most common examples of this in the environmental monitoring industry are SDI-12 and RS-485 Modbus RTU.
When connecting multiple devices that use the same communication protocol, it is necessary to individually assign unique addresses. If unique addresses are not assigned, all sensors on the same data bus with the same address will respond when the data logger sends a command. When multiple sensors respond at the same time, the data bus is corrupted and the responses become gibberish that the data logger cannot interpret.
Sensors only need unique addresses within their particular communication protocol. For example, an SDI-12 sensor and an RS-485 Modbus RTU sensor can both be assigned address 1. However, if there are two SDI-12 sensors connected, one will need to have its address changed to something unique like 2.
SDI-12 employs a standard means of changing a sensor’s address. Below is an excerpt pulled from our SDI-12 Protocol article that discusses changing an SDI-12 address using direct commands.
Change SDI-12 Address
Changes the sensor address from a to b. b must be from 0 to 9 and it should be an address that is not in use by any other SDI-12 sensor connected to the particular system.
RS-485 Modbus RTU Address
RS-485 Modbus RTU does not employ a standard such as this and the sensor specific manual should be referenced for the register information necessary to change the address.