--- title: "Bus Extender — Wiring and Bus Termination" description: "Wiring the Bus Extender: non-isolated and isolated ports, iGND, shielding and where the 100 Ω termination resistor goes by topology. Four wiring diagrams." url: "https://taphome.com/en/docs/wiring/bus-extender/" language: en lastmod: 2026-10-08 generator: hugo ai: topics: ["bus-extender","serial-bus","termination-resistor","galvanic-isolation","rs485"] questions: ["Does the Bus Extender have a built-in termination resistor?","Where do I put the 100 Ω termination resistor on a bus from the Bus Extender?","When should I use the isolated Bus 3 or Bus 4 port on the Bus Extender?","What is the iGND terminal on the Bus Extender for?","How do I wire remote modules powered from a second power supply through the Bus Extender?","How do I connect a Modbus RTU device with its own termination to the Bus Extender?","How do I connect the cable shield on an isolated Bus Extender port?"] --- # Bus Extender — Wiring and Bus Termination Bus 1–4 ports on the Bus Extender, when to use a galvanically isolated port, where the 100 Ω termination resistor goes depending on the topology, and four wiring diagrams. Last updated: 2026-10-08 The Bus Extender connects to the Core via USB and adds 4 RS-485 bus ports to the system — for the TapHome bus as well as Modbus RTU. Unlike the Core, it has no termination resistor built into any of its ports. Where the resistor goes depends on where the Extender sits on the line. ## Ports - **Bus 1, 2** — non-isolated (A, B) - **Bus 3, 4** — galvanically isolated (A, B, iGND) Use an isolated port if at least one of these applies: - A device on the bus is powered from a different source without galvanic isolation (for example a heat recovery unit). - The bus cable leaves the building where the Core is installed (for example to a garden house). Power the devices on the isolated bus from a separate power supply in that case. Other ways to reach another building, over fiber or with a second Core, are in [devices in another building](https://taphome.com/en/docs/wiring/remote-buildings/). - The cable is long and the modules on it draw more current, which causes a significant voltage drop on the cable. For low-consumption modules and cables up to 150 m, a non-isolated port is sufficient. On an isolated port, it is recommended to also connect iGND to the ground (GND) of the devices at the other end of the cable, so that A and B are at the correct voltage potential. Communication should work even without iGND connected. Up to 3 more Extenders can be chained through the Extender's USB port. The Bus Extender requires OS Image 2.11 or higher. ## Bus Termination The 100 Ω termination resistor goes between A and B at both ends of the line. The Core has it built in on every bus, the Bus Extender on none of its ports. Where to put it depends on the topology — the same for isolated and non-isolated ports: - **Extender at the end of the line:** 100 Ω on the Extender's A and B terminals and 100 Ω at the far end of the line. - **Extender in the middle of the line** (two branches leave its port): 100 Ω only at the ends of both branches, not on the Extender's terminals. ## Shielding On isolated ports (Bus 3, 4), the cable shield is connected to GND at the remote power supply (or to the GND of the module located near the supply); it is not connected at the ends of the cable. General shielding rules and cable selection are in [serial bus topology](https://taphome.com/en/docs/wiring/serial-bus-topology/). ## Wiring Diagrams ### Extender in the Middle of a Tree ![Bus Extender in the middle of a tree bus](https://taphome.com/documentation/wiring/bus-extender-tree.svg) Two branches leave the non-isolated Bus 1 port. The Extender sits between them, so no resistor goes on its terminals — 100 Ω is at the end of each branch. The modules are powered from the same power supply as the Core. ### Long Cable from an Isolated Port ![Long cable from an isolated Bus Extender port](https://taphome.com/documentation/wiring/bus-extender-long-cable.svg) The remote modules are powered from the power supply at the Core through a long cable. The isolated Bus 3 port carries A, B and iGND; at the remote end, iGND is connected to the GND of the modules. The Extender is at the end of the line, so 100 Ω goes on its terminals and at the far end. ### Remote Section from a Second Power Supply ![Remote bus section powered from a second power supply](https://taphome.com/documentation/wiring/bus-extender-second-power.svg) The remote modules have their own power supply. Only A, B and iGND leave the isolated port, with iGND connected to the GND of the second power supply; the +24 V of the two supplies is not connected. 100 Ω goes on the Extender's terminals and at the end of the line. ### Modbus RTU Device with Its Own Termination ![Modbus RTU device on an isolated Bus Extender port](https://taphome.com/documentation/wiring/bus-extender-modbus.svg) The device with its own power supply is on an isolated port, with iGND connected to its GND. 100 Ω goes on the Extender's terminals; on the device side, enable its built-in termination. If the device has none, put a 100 Ω resistor between its A and B terminals. Galvanic isolation of the bus with a separate isolator is described in [optical bus isolation](https://taphome.com/en/docs/wiring/optical-isolation/). ## Troubleshooting Common issues related to this topic: - [TapHome bus: measure Bus A and Bus B](https://taphome.com/en/troubleshooting/wiring/taphome-bus-voltage-a-b/) — Modules on the TapHome bus drop out, the bus analyzer shows errors or push buttons react late. A multimeter check of Bus A and Bus B against GND at the Core and at the modules shows a loose terminal, an unconnected wire or a faulty module. - [Modbus RTU: device drops out, no common GND](https://taphome.com/en/troubleshooting/wiring/modbus-rtu-missing-common-ground/) — A Modbus RTU device with its own power supply, such as a hybrid inverter, heat pump or heat recovery unit, goes into error for a few tens of seconds and its values have gaps, because only A and B are wired and its ground floats against the ground of the Core. [View all troubleshooting articles](https://taphome.com/en/troubleshooting/)