How Multiple RS485 Sensors Share One Bus: Addressing, Wiring and Polling

One advantage of RS485 is that multiple sensors can share the same A/B bus, but "can be connected in parallel" does not mean "casually connected in parallel". Address, upperology, power supply, and polling strategies all affect stability.

First principle: each sensor address must be unique

When two identical Slave ID s appear on the same bus, both devices may respond at the same time, causing conflicts, garbled codes, or no valid data. The safest thing to do is to connect each sensor individually, modify it to a unique address one by one, and then connect to the bus.

Second principle: the bus prefers to use the trunk structure

RS485 is more suitable for "trunk line + short branch line" routing, and it is not recommended to pull out a lot of long star branches from one central point. When the distance becomes longer and the number of nodes increases, it is necessary to pay attention to the terminal resistance, shielding, grounding and cable voltage drop.

Third principle: the power supply capacity is calculated by all devices together

Even if only a pair of A/B wires are used for communication, each sensor still needs a stable power supply. For multi-equipment items, the power supply should be calculated according to the total current, the longest distance and the wire diameter, and not only the nominal voltage of the power supply.

Don't blindly pursue high polling frequencies

The master device communicates with only one slave device at a time. Polling intervals should take into account sensor response time, number of bus nodes, and business requirements. Long-term meteorological and soil monitoring usually does not require millisecond updates, and it is more stable to reasonably reduce the frequency of polling.

FAQ

Q1: How many sensors can be connected to one RS485 interface?

A1: Theoretical quantity and actual available quantity are not the same thing. The actual project is affected by the transceiver load, line length, power supply, baud rate, upperology and polling cycle, and should be designed according to site conditions.

Q2: Do the registers of different types of sensors collide with each other?

A2: No. The register address belongs to each slave's own address space. Different sensors can each be read from register 0 or other starting position as long as the Slave ID is different.

Q3: Is it possible to use one turn multiple junction boxes?

A3: It can be used to improve the wiring space, but it is only an electrical branch and does not automatically solve the problem of address, terminal matching and bus upperology.