Modbus

Overview

Modbus is a communication protocol that provides a common language for devices and equipment to communicate with each other. The Modbus protocol provides the internal standard that 75F® devices use for parsing messages. The Modbus protocol is defined as a master/slave protocol, that is, a device operating as a master pings one or more devices operating as slave. During communications on a Modbus network, the protocol determines how each 75F® device knows the connected slave's address, recognizes a message addressed to it, determines the action to be taken, and extracts any data or information contained in the message.  

Modbus is an important communication protocol for different equipment in the building managing system (BMS) to communicate and exchange parameter value 75F® devices act as an integrator platform for third-party equipment that have Modbus enabled to communicate with our system. 75F® applications must in turn show them in a single unified view to see data on multiple dashboards/portals. 

In a Modbus RS-485 network, communication begins when the master device sends a query to a connected slave device. Each slave device is assigned a unique ID.  When the slave device receives the query, it can either act on the query or reply to the master device.

In the same way that RS-232 connectivity works, the slave and the master devices must be configured correctly. Parameters such as baud-rate and parity must be synchronized throughout the network.  

Modbus RTU 

The most commonly used form of Modbus protocol is Remote Terminal Unit (RTU) over RS-485. Modbus RTU is used in serial communication where data is transmitted in 8-bit bytes at baud rates ranging from 1200 bits per second to 115200 bits per second. Most of the Modbus RTU devices only support speeds up to 38400 bits per second. 

A Modbus RTU network has one master and one or more slaves. Each slave has a unique 8-bit device address or unit number. A packet sent by the master to the slave includes the address of the slave. The slave must then respond only if its address is recognized. Each data packet, whether a request or a response, begins with the device address or slave address, followed by the function code, and the parameters defining what is being asked for or what is being provided. 

Accessing Modbus Registers 

Modbus registers are associated with a function, and an offset within that function. The various functions define certain types of registers (bits, data registers, etc). While some registers are considered read-only and some may be written to. As a user, you need to know which register to request to obtain measures, and which registers to change to configure the device. At this level, the register addresses change from one device to another, but there are Modbus conventions useful to know.

There are four types of Modbus registers. The technical user guide of a Modbus equipment usually contains a list of all the available registers, listed by type. Each type of register corresponds to a range of register numbers, following this convention: 

• The coils, corresponding to on/off binary outputs, such as relays. The coils are numbered from 00001 onward. 
• The input bits, corresponding to binary inputs (read-only). The input bits are numbered from 10001 onward. 
• The input registers, corresponding to analog inputs (read -only). The input registers are numbered from 30001 onward. 
• The holding registers, corresponding to analog parameters, which can be changed. The holding registers are numbered from 40001 onward. 

Note: When one uses this convention, the first register of each category always ends by digit 1 (zero is not used). 

Sometimes, rather than indicating the unique register number, the device vendor indicates the type of register with the identifier of the Modbus function that allows you to read them: 

Modbus Network: In any Modbus RTU, there can be up to 32 slave devices connected via CCU. The slave devices can be energy meters or a combination of PAC, UPS, EM, or any other device. All these connected devices form a network. See Modbus Profiles.

Modbus TCP/IP 

Modbus TCP/IP is simply the Modbus RTU protocol with a TCP interface that runs on Ethernet. Modbus TCP/IP uses TCP/IP and Ethernet to carry the data 

of the Modbus message structure between compatible devices. The slave ID is still included and its interpretation varies by application but is not the primary means of addressing in TCP. Here the IP address is more important, for example, 192.168.1.100. 

Modbus TCP makes the definition of master and slave less obvious because Ethernet allows peer-to-peer communication. The definition of client and server is better known as entities in Ethernet-based networking. In this context, the slave becomes the server and the master becomes the client. 

Integrating Modbus-Protocol-Enabled-Third-Party-Device into 75F® System

The integration is through the RS-485 port on the CM board that connects to the central control unit (CCU). Integrating Modbus into 75F® system will affect the product at various levels - 75F® CCU, portals, and apps. 

The following illustration shows the workflow for Modbus integration with third-party equipment:

 The changes at various levels are:  

• CM 

Modbus connection to CCU is through the USB Port on the CM. For this, USB to RS-485 converter is used. 

• USB to RS-485 Converter

Waveshare brand of USB to RS-485 converter is used to establish connectivity between the Modbus devices and the CCU. This converter is easy to use and allows automatic transceiving. Due to its fast communication, stability, reliability, and safety, it is a good choice for connecting equipment and/or applications with high communication requirement. This is an industrial USB to RS-485 converter with original FT232RL and SP485EEN inside. It features embedded protection circuits such as lightning-proof, resettable fuse, ESD protection, and TVS diode, and so on.

For more information, click here.

CCU Modbus Processing  

CCU Modbus Engine  

The Central Control Unit needs to use the USB as serial connector acting as RS-485. The CCU Modbus engine acts as the Modbus master device and takes data from the various slave devices connected to it. The CCU Modbus engine follows the Modbus protocol for getting data from the third-party equipment that is configured to be integrated with 75F system. 

Configuring and Pairing Modbus

You must configure Modbus on CCU and then pair it. For more information, click here.

Modbus Support in Portals and Apps 

Once the Modbus modules are configured on the CCU and the Modbus parameters selected for visibility on the 75F® Setting screens, these Modbus zones can be viewed on portals and apps. 

The configured Modbus zones are:

• Available in Floor Planner to be dragged on to the floorplan for viewing in Heatmap. 
• Viewable on Heatmap as a zone supporting both mouse hover and click options.
  • Initially no predefined set of widgets for trend graphs will be available for Modbus zones as the registered parameters per equipment is dynamically set by the user. 

• Custom Visualization Builder customization is available to add widgets with data available for the zone as per equip points exposed and mapped by 75F®. As in CCU Zone Setting screen, the portals and apps also will show the Modbus zone settings. 
  • No schedule and vacation are applicable to these zones. 
  • All the Modbus registered parameters of the equipment that are set to be visible are shown with the latest value of the data on the equipment. 
    • If the registered parameters have read/write permissions, then you can change the value of the parameters with only the permissible allowed values.
    • If the registered parameters have only read-only permissions, then you can view the current status of the equipment. 
  • Modbus zones must be made available in tools for monitoring.

• Modbus zones points will be available in Portfolio Analytics Manager to create trend charts.

Modbus Profiles

This section explains the Modbus profiles that are currently supported by 75F®.

Precision Air Conditioning

Precision Air Conditioning (PAC) units are typically used in places such as server rooms or datacenters that require an environment with high levels of heat value to dissipate. PAC units provide constant temperature and humidity conditions to sensitive and expensive electronic equipment by controlling the ambient environment.

PAC units usually come with their own humidifier and dehumidifiers as well as inbuilt water leak detectors. PAC units can be broadly classified into air- or water-cooled direct expansion (DX) version and chilled water version. Even within the DX type, the compressor can be fixed scroll type or variable scroll type. Depending on the capacity of the units, PAC can be single stage or multistage units. 

The following table lists all critical points to be mapped/monitored and their Modbus registers for Flaktwood:

Water Leak Detection System

Water Leak Detection (WLD) systems are used to detect leaks from equipment such as water pipes or areas such as bathrooms, wet rooms, kitchens and so on. To prevent any damage to the critical rooms due to water leakage, a dedicated alarm system is provided. This allows facilities managers to take corrective action before any substantial damage occurs.  

WLD system is a Modbus-based system that sends out alerts when activated. WLD system can be single zone or multiple zones based on the configuration.    

The following table lists all critical points to be mapped/monitored and their Modbus registers for C-System:

The following image shows how these parameters appear on the CCU:

Rodent Repellent System

Rodent Repellent Systems (RRS) are used to remove pesky rodents from secure areas such as hub rooms or server rooms where they cause damage to cables by chewing them. Any replacement of the damaged cables is not only disruptive to the daily operations but is also expensive.

RRS is a Modbus-based system controls underfloor transducers by continuously emitting ultrasonic frequencies that keep the rodents away. The Modbus details for the system are as follows.  

The following table lists all critical points to be mapped/monitored and their Modbus registers for C-System:

The following table lists all critical points to be mapped/monitored and their Modbus registers for Masers:

Uninterruptible Power Supply

Uninterruptible Power Supply (UPS) is an electronic device used to supply power to devices such as computers, telecom equipment etc. so no critical data is lost whenever there is power outage. Usually, UPS powers on critical equipment present in a facility and requires alerts and status updates regarding the power and backup parameters.    

a. The following table lists all critical points to be mapped/monitored and their Modbus registers for Vertiv UPS Model: ITA2 (30KVA):

b. The following table lists all critical points to be mapped/monitored and their Modbus registers for
Vertiv eXM: Using Vertiv Unity Card (80 KVA):

c. The following table lists all critical points to be mapped/monitored and their Modbus registers for Vertiv APM: Using Vertiv Unity Card (400 KVA):

d. The following table lists all critical points to be mapped/monitored and their Modbus registers for Fuji Consul Neowatt 700X:

Variable Refrigerant Flow

Variable Refrigerant Flow (VRF), also known as Variable Refrigerant Volume (VRV) is an HVAC technology that uses refrigerant as the cooling and heating medium. VRF HVAC systems use variable motor speed and variable refrigerant flow to heat and cool unlike standard units that use a simple on/off operation.  

Variable refrigerant flow (VRF) is integrated using Modbus protocol so you can monitor and control VRF. 

a. The following table lists all critical points to be mapped/monitored and their Modbus registers for Intesis Asco:

The following image shows how these parameters appear on the CCU:

b. The following table lists all critical points to be mapped/monitored and their Modbus registers for Bluestar:

Energy Meter 

Energy meters (EM) are widely used to measure the power consumed by the device.  

Energy meters with an integrated Serial RS-485 Modbus interface allow direct reading of all relevant data, such as energy (total and partial), current, and voltage for every phase and active and reactive power for every phase and for the three phases. 

a. The following table lists all critical points to be mapped/monitored and their Modbus registers for Schneider IEM 3150:   

b. The following table lists all critical points to be mapped/monitored and their Modbus registers for ElmeasureMulti-Function Meter: