---
title: Testing a Modbus TCP connection
slug: how-tos/f44B-testing-a
docTags: XvgE82X7SfoIRBJdtS4Hy
createdAt: 2025-09-22T18:48:23.971Z
---

## Step 1: Install a Modbus client

You need a tool to connect to and browse your Modbus TCP device. Here are some free and common options:

| Client Name                      | Platforms | Link                                                                   |
| -------------------------------- | --------- | ---------------------------------------------------------------------- |
| ModScan / ModSim                 | Windows   | [Download](https://www.win-tech.com/)                                  |
| Schneider Modbus Tester          | Windows   | [Download](https://www.se.com/us/en/faqs/FA180037/)                    |
| Eurotherm iTools (for Eurotherm) | Windows   | [Download](https://www.eurotherm.com/download/eurotherm-itools-v9-87/) |

Each client lets you manually connect to a Modbus TCP device, specify a unit ID (slave ID), function code, register address, and data type, then read or write values.

## Step 2: Prerequisites

Before testing, make sure you:

- Know the **Modbus slave address** (unit ID). Valid slave addresses are `01` to `F7` (1 to 247). Address `00` is the broadcast address and is write-only, so it cannot be used to read.
- Know the **register address** and **data type** you want to read.

**Example**:
Reading a 32-bit float from register 40001 on slave 01.

## Step 3: Test the connection

1. Open your chosen Modbus client.
2. Enter the IP address and TCP port (default 502).
3. Specify:
   - **Slave ID:** e.g. 01
   - **Function code:** typically 3 (Read Holding Registers)
   - **Start address:** e.g. 40001
   - **Quantity:** number of registers to read
4. Click **Connect** or **Read** to view the result.

## Troubleshooting Modbus TCP connection

### 1. Slave ID format

- The **Slave ID** in the collector is in **hexadecimal notation**, not decimal.
   **Example**:
  - Device ID shown as 10 decimal = 0x0A hex.
  - Use an [online converter ](https://www.rapidtables.com/convert/number/hex-to-decimal.html)

Slave IDs must always be **two digits**. For example, 0A instead of A.

### 2. Unknown Slave ID

If the slave ID is not known:

- Set **FF** (255). In Modbus TCP this is the unit ID for a device that is addressed directly rather than through a gateway, so a single connected device answers on it.
- The device answers with its own data, which confirms the connection before you know its real unit ID.

Use FF **only if one device** is connected. A Modbus TCP gateway fronting several serial slaves needs the real unit ID of each one.

:::hint{type="warning"}
Never use FF on a gateway fronting several slaves: it does not address any one of them. Broadcast (`00`) is write-only and never returns a reply, so it cannot be used to test a connection either.
:::

### 3. 1-based vs 0-based addressing

- The Go Modbus library used by the collector uses **0-based indexing**.
- **Example**: If documentation says **register 1006**, use **address 1005** in the collector configuration.

### 4. Modbus TCP vs Modbus RTU

- The **Factry Modbus collector** supports **Modbus TCP only**.
- If you have a Modbus RTU (serial) device, use an **external TCP-to-RTU gateway** or **bridge software**.
  Example: Moxa MB3180 or ModbusPal emulator.