---
title: "Serial Bus Topology"
description: "How to wire the TapHome bus: linear, tree or star topology, why a ring is not allowed, 100 Ω terminator at the farthest point, max 32 modules, cables."
url: "https://taphome.com/en/docs/wiring/serial-bus-topology/"
language: en
lastmod: 2026-10-08
generator: hugo
ai:
  topics: ["serial-bus","wiring-topology","termination-resistor","cable-requirements","rs485"]
  questions: ["What bus topologies are supported for TapHome modules?","What is the maximum number of modules on one TapHome bus?","Where should I place the 100 Ω termination resistor?","Why is ring topology prohibited and how do I fix existing ring wiring?","Which cables are recommended for TapHome bus - Cat5E, Cat6, or RS-485?","Why must wires A and B form a twisted pair?","How do I connect the cable shield when using Bus Extender optical isolation?"]
---

# Serial Bus Topology

Method of connecting components in different types of buses - linear, tree, star, and ring topology. Recommendations for the correct connection of TapHome modules.

Last updated: 2026-10-08

The methods of connecting components in different types of buses.

### Linear Topology

![Linear topology](https://taphome.com/documentation/wiring/serial-bus-topology-linear.svg)

The basic bus configuration consists of a line terminated at the furthest point with a 100 Ω termination resistor (the Core package includes three, one for each bus).

**Key characteristics:**
- Maximum of 32 modules on one bus
- Terminator at the furthest point
- It is not necessary to maintain spacing between modules

### Tree Topology

![Tree topology](https://taphome.com/documentation/wiring/serial-bus-topology-tree.svg)

This branched configuration is permissible within the system. The rule for the placement of the terminator remains critical – it must be at the physically furthest point.

**Important note:** Branching must be done carefully and should be minimized. Excessive branching can cause undefined states in which peripherals disconnect from the bus, leading to system malfunctions.

### Star Topology

![Star topology](https://taphome.com/documentation/wiring/serial-bus-topology-star.svg)

Commonly used in smaller control systems and residential applications with short distances between devices. This topology requires careful attention to cable routing, bundling, and other bus requirements.

**Disadvantage:** The system may be more prone to communication failures and interference. When using this topology, pay increased attention to the quality of wiring and minimize the length of branches.

### Ring Topology 🚫

![Ring topology](https://taphome.com/documentation/wiring/serial-bus-topology-ring.svg)

The ring configuration is **strictly prohibited** for all two-wire communication buses.

**Solution for existing ring wiring:** If the wiring is already completed in a ring configuration, connect one end to a 100 Ω termination resistor instead of the Core output. This will create two parallel lines that maintain functionality in case of cable damage.

### Multiple Power Sources

![Linear topology with two sources](https://taphome.com/documentation/wiring/serial-bus-topology-dual-power.svg)

Over extended distances, multiple DC sources can power control modules, provided that their grounds (GND) remain interconnected.

**Note:** The necessity of this implementation depends on the distances in the project and the choice of cable type.

### Wiring Technical Specifications

#### Recommended Cables

For industrial RS485 and Modbus RTU applications, cables meeting EIA RS-485 standards with a 120 Ω impedance are required.

**For TapHome control blocks, creating a bus with the following is sufficient:**
- Cat5E
- Cat6
- Cat6A and higher

Wiring certified for RS-485 networks is not required.

> [!WARNING]
> **Twisted Pair**
>
> It is **very important** that wires A and B form a **twisted pair**. This twisting significantly reduces electromagnetic interference and ensures reliable communication on the bus.

#### Shielding

Shielded cabling is not strictly required, although it is recommended in environments with higher electromagnetic interference.

**Connection of cable shield:**
- The cable shield must be connected to GND at the Core module
- At the opposite end, it is not connected anywhere

**In case of optically isolated buses on the Bus Extender:**
- The shield is connected to GND at the remote source (or the GND of the module located near the source)
- At the ends of the cable, it is not connected

Ports, termination and wiring diagrams of the Bus Extender: [Bus Extender](https://taphome.com/en/docs/wiring/bus-extender/).

> [!WARNING]
> Avoid mixing different types of buses in unshielded multi-pair cables to minimize mutual interference.

#### Termination Resistor

- **Value:** 100 Ω at the furthest point of the bus
- **Multiple terminators:** Allowed in tree topology if you are unsure of the furthest point.

## 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/)