Modbus data scanner. What is Modbus and what Modbus scanner to choose for monitoring

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Modbus scanner

What is Modbus?


Modbus is a serial communication protocol that was developed by Modicon in the late 1970s. Its primary purpose is to allow communication between programmable logic controllers (PLCs). The Modbus protocol is a master / slave protocol. This means one device serves as the Modbus Master - polling one or more other Modbus Slaves. The Modbus master / slave protocol is a one to many relationship where one master can connect with up to 247 slaves. The device serving as the slave waits for the master to request information from it.

There are multiple variations of the Modbus protocol. We'll talk about its serial (RS-232 or RS-485) version - Modbus RTU. It is an open protocol based on master-slave architecture. This protocol is rather popular thanks to its reliability and simplicity. It is widely implemented in Building Management Systems (BMS) and Industrial Automation Systems (IAS).

The messages sent through Modbus RTU are a simple 16-bit CRC (Cyclic-Redundant Checksum), which ensures reliability. The basic 16-bit Modbus RTU register structure is used in floating-point, tables, ASCII text, queues, etc.

As mentioned above Modbus RTU works on master-slave architecture and provides communication for devices connected via networks and buses and delivers services specified by function codes. This protocol is positioned at level 7 on the OSI model.

Modbus scanner software


To create a Modbus application data unit one has to initiate a Modbus transaction. This is a function that will inform a server what action is expected from the latter. The function code field is coded into one byte. Valid codes are from 1 to 255 bytes, with 128 to 255 reserved for exception responses.


Modbus data scanner

Modbus scanner software (aka Serial Port Monitor) helps to log and decode Modbus RTU and ASCII messages.
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Serial Port Monitor

Log and analyze serial port activity
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Serial Port Monitor

In some cases it is needed to monitor the whole data flow through a port. It can be to troubleshoot, test, etc. It is rather convenient and efficient to do with a special Modbus scanner software - a good example will be Serial Port Monitor developed by Eltima Software. You can download the software from developer's official website and start a monitoring session. Look for Modbus RTU and ASCII data that you can scan and filter.

This Modbus Scanner tool allows to:

  • Monitor several ports in one session;
  • Intercept and analyze Modbus data;
  • View the scanned data in different modes;
  • Emulate the data sending.

Eltima Modbus data scanner (Serial Port Monitor) can also reproduce a monitoring session in case you want to test a certain port behavior.

FAQ
Modbus is freely available for download and carries no licensing fees. It is a widely used industry standard for data transportation between various machines. A common use of Modbus is to allow the monitoring of field devices. These devices are able to send information back to a controlling device for monitoring, programming and maintenance.
Once a connection between the master and slave is established, the Master initiates the conversation. The slave interprets the message and responds accordingly. All the data that is passed between master and slave is sent as a series of ones and zeroes or bits.
How does Modbus work
The bits are represented as an electrical voltage - where a zero is depicted as a positive voltage, while a one is represented by a negative voltage. The speed of transfer is as fast as the slowest element in the communication chain, but a typical speed is about 9600 baud (bits per second).
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Serial Port Monitor
Log and analyze serial port activity
4.8 rank based on 41+ users
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