Creating a Modbus TCP Tag

After creating a tag on a Modbus TCP device, you need to tell Koios which register to read or write and how to interpret its value. You can configure these fields during creation or afterwards on the tag's Configuration tab.

Use the register browser to discover data points

On the Configuration tab, click Browse to scan registers on the device. The browser lets you read raw register values and test different data type interpretations before committing to a configuration.

Protocol Fields

Register Type

The type of Modbus register this tag reads from.

Type	Description	Access
Coil	Single-bit read/write registers — typically used for on/off controls	Read/Write
Discrete Input	Single-bit read-only registers — typically used for digital sensor states	Read Only
Input Register	16-bit read-only registers — typically used for analog measurements	Read Only
Holding Register	16-bit read/write registers — the most common type for configuration and data	Read/Write

Required

Coil and Discrete Input auto-select Boolean

When you select Coil or Discrete Input, the data type is automatically set to Boolean and locked, since these register types are inherently single-bit.

Register Address

The address of the register on the device.

Required
Range: 0–65535
If the device has one-based addressing enabled, the display adds 1 to the address for readability, but the underlying communication always uses the 0-based value

Data Type

How Koios interprets the raw register value(s).

Data Type	Registers Used	Description
Boolean	1	Single bit — used with Coil/Discrete or bit extraction from a holding register
Int16	1	16-bit signed integer (-32,768 to 32,767)
UInt16	1	16-bit unsigned integer (0 to 65,535)
Int32	2	32-bit signed integer — reads two consecutive registers
UInt32	2	32-bit unsigned integer — reads two consecutive registers
Float32	2	32-bit floating point — reads two consecutive registers
Float64	4	64-bit floating point — reads four consecutive registers

Required
Multi-register types (Int32, UInt32, Float32, Float64) read consecutive registers starting from the configured address

Byte Swap

Reverses the byte order within each 16-bit register.

Default: Off
Enable this when the device stores values in a different byte order than expected — common with some manufacturers

Word Swap

Reverses the order of registers for multi-register data types (Int32, UInt32, Float32, Float64).

Default: Off
Only visible when using a multi-register data type
If a 32-bit value reads as a nonsensical number, try toggling this setting

Byte order varies between manufacturers

There is no universal standard for byte and word order in Modbus. If your values look wrong (very large numbers, NaN, or values that don't match the device's display), try different combinations of byte swap and word swap until the value is correct. The register browser can help you test this.

Bit Number

When using Boolean data type with an Input Register or Holding Register, this specifies which bit within the 16-bit register to extract.

Optional — only shown when data type is Boolean and register type is Input or Holding
Range: Bit 0 (least significant) through Bit 15 (most significant)
Leave empty to use the entire register value as a boolean (0 = false, non-zero = true)

Using the Register Browser

The Modbus register browser lets you scan and interpret register values directly from the device.

Open the tag's Configuration tab and click Browse
Select the register type and enter a start address and count
Click Scan to read raw register values from the device
Use the interpretation tools to test different data types, byte swap, and word swap combinations
When you find the correct interpretation, apply it to populate the tag's configuration

This is especially useful for unfamiliar devices where the register map isn't well documented.

After Configuration

Save the tag configuration
Test the tag — click
Test
to verify Koios reads the correct value (see Testing a Tag)
Enable the tag — flip the enable switch to start collecting data

Enable the device first

Tags only collect data when their parent device is also enabled and running.

Creating an OPC-UA Tag Creating an EtherNet/IP Tag