RS485 interfaces

Last modified: 2026-04-20

The bluelog is equipped with two RS485 interfaces (RS485‑1 and RS485‑2) for connecting bus devices such as inverters, energy meters, power analyzers, and string monitoring systems. Sensors and meters can be queried together if they communicate via Modbus.

This page provides general tips for using these interfaces. For wiring of selected individual device types, see Device connection plans.

Communication protocol rules

• Each RS485 interface supports only one protocol (e.g., Modbus).

• Devices from different brands or categories with a Modbus RTU protocol can be mixed on the same bus line. However, this is not recommended because it reduces the performance and quality of the Monitoring and Power Control functions.

• If devices from different brands or categories share an RS485 interface, the baud rate and protocol frame of the RS485 settings must be identical.

• The bluelog operates exclusively as the Master on the bus.

Bus limits and topology rules

• Observe the maximum permitted number of bus devices.

• The order of devices along the bus is irrelevant.

• Use a repeater for every 32nd bus device or long cable runs.

• The bus should be cabled with a twisted and shielded pair of wires.

• The shield of the bus cable must be grounded at one end only; the bluelog has no grounding terminal.

• Route AC and DC cables separately from the bus wires.

• Do not switch the bus signal wires.

• RS485 A/B wire labels vary by manufacturer. The + and – markings are unambiguous.

• To prevent reflections, the bus must be terminated with a parallel terminator.

Connect multiple devices to the bus

You can use the following wiring topologies to connect multiple devices to the bus.

Daisy chain

With daisy chain wiring, different devices can only be queried jointly if they use the same communication protocol and the same serial communication parameters (baud rate, data bits, parity, stop bits). The first and last device on the bus must be terminated with a resistor. The bluelog has integrated terminating resistors, which can be turned on and off via the termination switches.

Star wiring

Star wiring uses the 6 PORT RS485 HUB S (product no. 421.641) to split the bus into several lines.

Various devices can only be queried together if they communicate with the same protocol and the serial communication parameters are identical (baud rate, number of data bits, parity, stop bits).

• Each bus string can have a maximum length of 1200 m.

• All devices on the same bus string are wired together in the daisy chain.

• The first and last device of each bus string must be terminated by a resistor.

• The fieldbus extension box has integrated terminating resistors at each interface, which are permanently active.

• The total number of bus devices may not exceed the maximum number of bus devices.