Data acquisition is the process of sampling signals including Digital and Analogue that measure physical conditioners and then converting the samples to numeric values.
Modbus Data Loggers
These devices can monitor third-party systems using one of two Modbus protocols: Modbus TCP/IP or Modbus RTU. Modbus is used in industrial automation applications to connect, monitor and control programmable logic controllers (PLCs), third-party systems and compatible sensors. Information is transmitted in a compact binary format. Modbus TCP/IP runs over Ethernet and offers higher speeds, virtually unlimited distance and support for complex networks. Modbus RTU uses a serial connection (RS-485/RS-232) and is suitable for shorter-distances, and is a lower-cost networking device. The two forms of the protocol also differ in their error checking. Modbus TCP/IP uses the error handling build-into the Transmission Control Protocol (TCP) where as Modbus RTU relies on Cyclic Redundancy Check (CRC) error checking.
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What is Modbus?
Modbus is a relatively simply and widely used industrial and building systems communications protocol. Typical systems that use Modbus include PLCs, controllers, data loggers and other general building systems including HVAC (heating, ventilation and air conditioning) as well as other systems that can provide Modbus interfaces such as emergency lighting, general lighting, lifts, access control, standby power generators, DC power systems and uninterruptible power supplies. Modbus is the most commonly used protocol to collect data into a central logging device or SCADA (Supervisory Control and Data Acquisition) platform.
Modbus Principles
Modbus works on a master-slave (client-server) model. The Master/Client is usually a data logger or PLC or the computer requesting the data and information. The Slave/Sver is the device providing the information such as a power meter, flow meter or sensor (temperature, humidity, flood, vibration or dust particles). The master sends a command: send me the register X and the save responds with the value. X being the register within the Modbus device.
Modbus Registers
Registers are unique and defined storage areas for the data within a slave device. Each register has an address (number), stores a value and can hold up to 16 bits (one 16-bit word) of data.
There are four logical types of Modbus register:
| Type | Name | Read/Write | Typical Use |
|---|---|---|---|
| 0xxxx | Coils | Read/Write | Digital outputs (ON/OFF) |
| 1xxxx | Discrete Inputs | Read only | Digital inputs |
| 3xxxx | Input Registers | Read only | Analog inputs (sensor readings) |
| 4xxxx | Holding Registers | Read/Write | Configuration & measured values |
For data logging the most commonly used are the input registers (3xxxx) and Holding registers (4xxxx).
Third-party systems manufacturers produce Modbus Register lists for the information then can provide for monitoring. The Projects Team at Server Room Environments can help you to collect this information in your chosen device. The selected monitoring device will have an RS-485/Mobus port and will use virtual sensors to map to the data and registers being read.
For more information on our range of M-bus compatible data logging devices please contact our Projects Team.
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