---
title: "TapHome bus modules stop responding or show errors — measure the voltage on Bus A and Bus B"
description: "TapHome bus modules drop out or show errors in the bus analyzer? Measure Bus A and Bus B against GND at the Core, the modules and the Bus Extender and find the fault."
url: "https://taphome.com/en/troubleshooting/wiring/taphome-bus-voltage-a-b/"
language: en
lastmod: 2026-11-05
generator: hugo
---

# TapHome bus modules stop responding or show errors — measure the voltage on Bus A and Bus 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.

- **Service** You need the Service permission (typically your installation partner).

## How you recognise it

Modules on the TapHome bus communicate badly or not at all:

- Several modules stop responding at the same time, or one module keeps dropping out
- Bus analyzer shows communication errors of some modules in red
- The LED of a module blinks once — it receives invalid packets
- Push buttons and other modules on the bus react with a delay of up to a minute

## Cause

The Bus A and Bus B line is not in a good state. Most often a terminal is not tightened, Bus A or Bus B is not connected at a module, or a faulty module pulls the line. A faulty module can still communicate itself while it disturbs the other modules on the same bus.

## Solution

### Step 1: Find the affected modules

Service installation partner

**In the app:** Settings → Hardware

Open **TapHome Bus** and its Bus analyzer. Note the modules shown in red and the bus they are connected to.

### Step 2: Measure Bus A and Bus B against GND

Service installation partner

Set the multimeter to DC voltage and measure while the Core is running. Put the black probe on GND and the red probe on Bus A, then on Bus B:

1. At the Core: GND of its power connector against Bus A and Bus B of the bus port (BUS1 to BUS3)
2. At the module: GND of its red Power connector against Bus A and Bus B of its blue Bus connector — always against GND of the same module
3. On the [Bus Extender](https://taphome.com/en/docs/wiring/bus-extender/): Bus 1 and Bus 2 against GND; Bus 3 and Bus 4 are isolated, measure them against their own iGND terminal
4. A healthy bus shows roughly **1.2 to 2.0 V** on both Bus A and Bus B, at the Core and at the modules. The values are approximate.

> At each module also check its power supply: between +24V and GND of the red Power connector there must be 24 V DC ±10 %.

### Step 3: Fix it according to the result

Service installation partner

1. The Core is fine and one module is out of range: tighten the Bus A, Bus B and GND terminals at that module and at the modules before it, and check that both Bus A and Bus B are connected
2. The values are out of range at the Core too, on the whole line: a faulty module may be pulling the bus. Disconnect the Bus connectors of modules or whole branches one by one, starting in the middle of the line, and measure at the Core after each step. When the values return to the range, the fault is in the part you disconnected last — replace the faulty module

## If it still does not work

- Check the 100 Ω termination resistor at the far end of the line — see [serial bus topology](https://taphome.com/en/docs/wiring/serial-bus-topology/)
- Reset the error statistics in Bus analyzer and watch whether new errors appear
- Contact TapHome support — they can read the communication errors of each module from the log of the Core

## Related Documentation

- [Serial Bus Topology](https://taphome.com/en/docs/wiring/serial-bus-topology/)
- [Bus Extender — Wiring and Bus Termination](https://taphome.com/en/docs/wiring/bus-extender/)
- [Bus Analyzer](https://taphome.com/en/docs/setup/bus-analyzer/)

## Related issues

- [Push button reports every press as a long press or several short presses — the input is active at rest](https://taphome.com/en/troubleshooting/wiring/push-button-input-active-at-rest/)
- [DALI lights switch on by themselves or jump to another brightness — the DALI bus is disturbed](https://taphome.com/en/troubleshooting/wiring/dali-lights-switch-on-by-themselves/)
- [Modbus RTU device with its own power supply drops out now and then — the RS-485 line has no common ground (GND)](https://taphome.com/en/troubleshooting/wiring/modbus-rtu-missing-common-ground/)
- [External sensor not communicating after replacement](https://taphome.com/en/troubleshooting/hardware/ext-sensor-not-communicating-after-replacement/)

Last updated: 2026-11-05