Modbus RTU Wiring Examples
Connect Modbus RTU devices to TapHome with two A/B conductors on an RS-485 bus terminated by a 100 Ω resistor, each with a unique address 1–247.
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A Modbus RTU device connects to TapHome over an RS-485 bus using two conductors, A and B, and the bus is terminated with a 100 Ω resistor unless the product already has one built in. This page gives rules and examples for wiring such devices, including addressing, communication parameters and cable requirements. You need devices that communicate via the Modbus RTU protocol, each with a unique address in the range 1–247 and all set to the same baud rate, parity and number of stop bits. The result is a Modbus RTU bus with TapHome control blocks, for which cables such as Cat5E or Cat6 can be used.
Examples of Modbus RTU cabling
For devices connected to the Modbus RTU line, different rules apply when connecting devices. Devices are connected using two A / B conductors.
Implementing a GND connection from a 24V DC power supply is not required. However, if devices require this connection according to the manufacturer’s specifications, this requirement should be implemented.
Device addressing
Since manufacturers typically configure their peripherals to address 1 by default, each device should be assigned a unique identification address in the range 1–247 when connecting them sequentially.
Devices with the same address in the system will not function.
Bus termination
The bus should be terminated with a 100 Ω resistor. Some products already have this resistor built in; in that case, it does not need to be mounted on the connection terminals.
Types of devices
Devices that communicate via the Modbus RTU protocol are mostly heat recovery systems, air conditioning units, energy meters, relay output modules, and input modules.
Communication parameters
For proper Modbus communication, all peripherals must be configured with the same communication parameters: baud rate, parity, and number of stop bits.
Cabling requirements
For standard RS485 and Modbus RTU wiring used in industrial installations, a cable type that meets the EIA RS-485 requirements with an impedance of 120 Ω is specified.
To create a Modbus RTU bus with TapHome control blocks, it is possible to use cables such as Cat5E, Cat6, Cat6A, and others.
Cabling specifically certified for RS-485 networks is not required.
Also, shielded cabling is not strictly required, either overall or for individual pairs.
In cables that do not have individually shielded pairs, it is not recommended to run two different networks such as RS-485 and 100/1000 Mbit LAN, or other bus networks (intrusion alarm systems, fire alarm systems, Modbus RTU, etc.).
Wiring diagram
See also — Surge Protection, Optical Bus Isolation, Modbus, Devices in Another Building.