Modbus

Modbus is a serial communication protocol developed in 1979 by Modicon (now Schneider Electric) for use with programmable logic controllers (PLCs). It has become a standard protocol for industrial communication due to its simplicity and reliability.

In essence, Modbus allows devices to communicate over various types of networks, including serial lines (Modbus RTU and Modbus ASCII) and Ethernet (Modbus TCP). It operates on a master/slave or client/server architecture, where the master (or client) sends requests to the slaves (or servers), which then respond with the requested data.

Modbus is widely used in industrial environments for connecting and transmitting data between devices like sensors, controllers, and computers. Energy meters are a classic example: almost every industrial energy meter speaks Modbus.

Basics

Modbus contains registers with addresses. Each register address contains 8 bits. An 8 bit value can not be more than 255. To overcome this low number, in Modbus you can combine multiple registers to form a number.

• Two 8 bit registers become 16 bit
• Four 8 bit registers become 32 bit
• Eight 8 bit registers become 64 bit

This represents:

Or for unsigned bits this represents:

If you read from address 1 and read a 64 bit value, UpBlue will read register 1 to 9 and combine this to one value.

Modbus supports 4 different types of registers:

1. Coil
2. Discrete
3. Holding
4. Input

Coil and Discrete registers are on/off 1 bit registers. Holding and Input can contain more than 1 bit, namely 8 bits.

Create a Connection with Modbus

Connection details

Fields

Connection name

This is the name given earlier when creating the connection. This is only for identification in the UpBlue Management Environment.

Address

This is the address of the Modbus client. This can be an IP address for Modbus TCP or UDP, or a COM port for Modbus RTU. For Modbus TCP or UDP, add a port. Normally this is port 502.

No need to add http://, https:// or something else. Based on the selected Protocol, the right prefix will be used.

Example of a Modbus address: 192.168.0.152:502, localhost:502 or COM6.

Slave ID

The slave id the client is running on. If there are multiple clients (slaves) on the same IP address, don’t create multiple connections. Override the slave ID in the rule.

Protocol

UpBlue supports the following protocols:

• TCP (default)
• UDP
• RTU (serial)
• RTU over TCP

For RTU, some extra configuration like speed, data bits, stop bits and parity needs to be set. You can find these values in the documentation of the device. When a known device (brand and model) is selected, these settings are filled in for you.

Security

Because Modbus is so simple, username, password or other security measures are not part of the standard.

Endianness and Word order

In Modbus, the order bytes are sent in differs per manufacturer. Endianness and Word order refer to the order in which bytes are transmitted over the network. Modbus typically uses big-endian format and HIGH word first. There are four possible combinations for a 32 bit number:

1. ABCD
2. BADC
3. DCBA
4. CDAB

In this example each letter represents 8 bit. 16 bits is seen as a pair. Byte swapping (Endianness) is swapping the two 8 bits (2x one byte). Word swapping (word order) is swapping the 16 bit pair in a 32 bit group.

When using 16 bits, Word order does not apply. When using 8 bit, both word order and endianness do not apply.

When the Endianness and Word order are not known, you can find the order by trial and error. You do need to know what registers there are and what values they represent.

Start with a 16 bit number to find out the Endianness, then move to a 32 or 64 bit number to find the word order.

Test connection

Rules

A rule is configuration that will retrieve a specific set of information from the Modbus client. It instructs the Collector to retrieve this information and send it to the Cloud.

Fields

Name

The name is only used for identification in the UpBlue Platform.

Tag

The tag name, for example energy_total or speed. Read more about tags.

Register

Modbus supports 4 different types of registers:

1. Coil
2. Discrete
3. Holding
4. Input

Coil and Discrete registers are on/off 1 bit registers. Holding and Input can contain more than 1 bit, namely 8 bits.

Datatype

Select the datatype. You need to get the datatype from the client documentation. Integer, unsigned integer and boolean in 16, 32 or 64 bit, float in 32 or 64 bit, or text (string).

Boolean datatype

Some manufacturers use input or holding registers as coil/discrete register. For example a 16 bit number is used as 16 times true/false.

For example: This is a representation of a 16 bit number: 1000000000000000 Each number is one bit. In this case only the first bit is “on”. If the mapping is created as this, two tags are created. machine_on with value 1 and machine_error with value 0.

If the value changes to 1100000000000000, the machine_error tag will change to 1.

The tag on the rule will be ignored.

Address

The address to read from.

Poll rate

The time between measurements, in seconds.

Transform values

Transformations clean and convert the values of this rule before they are stored. Every new rule starts with a Deduplication transformation, so unchanged measurements are not stored over and over. Counters, timers, text to value and more are also available. Read all about transformations here.

Override Slave ID

When multiple slave devices are on the same IP address or COM port, the rule can be pointed to the right device. If the device has another Endianness or another Word order, this can also be changed here.

Labels

You can override or add labels per rule. Read more about this here.

Test rule

A new window will open with the test results.