Main points about RS485. How to log RS485 activity.

Olga Weis
RS485 data logger

RS485 is a common communications standard that is widely used in data acquisition and control applications. One of its main advantages is that it allows putting several RS485 devices on the same bus which makes it possible for multiple nodes to connect with each other. This article covers some of the most commonly asked aspects of RS-485 communications and tells what RS485 communication is and how to efficiently analyze it.

Contents


  1. What is RS485?
  2. Software solution: RS485 data logger
  3. What is RS485 Modbus protocol?

What is RS485?


RS-485 interface (also referred to as EIA / TIA-485) is a standard of the physical layer of communication. The physical layer is the communication channel and the signal transmission method (level 1 of the OSI open system interconnection model).

Communication network built on the RS-485 interface consists of transceivers connected by a twisted pair (two twisted wires). The basic principle of RS-485 interface is differential (balanced) data transmission. That means one signal is transported over two wires. With that, one wire of the pair transmits the original signal and the other one transports its inverse copy.

How far can you run RS485?


As a result of differential signal transmission there is always a potential difference between the wires. This ensures high resistance to common mode interference. In addition, the twisted pair may be shielded, which ensures the protection of transmitted data. All this allows sending data over long distances at relatively high speeds, which can reach 100 kbits/s at 4000 feet. 4000 feet or about 1200 meters is the maximum cable length in RS-485 communications. A general guideline, however, is that the product of line length (in meters) and the data rate (in bits per second) should not be more than 108. For instance, a 20-meter cable allows a maximum data rate of 5 Mbits/s.

What is RS485 communication protocol?


When RS485 communication line is ready for operation at the physical level, it's time to think about the data transfer protocol - an agreement between the system's devices on the format of the data packets transmission.

By the nature of RS-485 interface, RS-485 devices cannot transmit and receive data at the same time, as it leads to a conflict of transmitters. Therefore, the deterministic behavior is mandatory to avoid collisions of data packets.

In RS485 communication protocol, the commands are sent by the node which is defined as a master. All other nodes connected to the master receive the data over RS485 ports. Depending on the information sent, zero or more nodes on the line respond to the master.

With that said, this type of communication is not the only possible way to exchange data over RS485 protocol. There are some other implementations of RS485 networks where every node can start the data transfer on its own. However, the risk of possible data loss is higher in such networks.

Software solution: RS485 data logger

Working with serial apps and devices, specialists often rely on dedicated software, like RS485 Analyzer by Eltima, to sniff and analyze serial port activity, detect errors, compare received data, etc.

RS485 Protocol Analyzer

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Serial Port Monitor [Professional Edition]


Requirements: Windows (32-bit and 64-bit): XP/2003/2008/ Vista/7/8/10, Windows Server 2012. 9.16MB free space. Version 7.0.342. (16th Jan, 2018). 4.8 Rank based on 41+ users, Reviews(75)
Category: Serial Port Software
RS485 Analyzer is a professional utility to identify problems in real time and resolving them straight away.
RS485 Tester - RS485 test software

Serial Port Monitor works as an RS485, RS422 sniffer and makes it possible to analyze data from several COM ports simultaneously. The solution is also useful when it's required to connect to an RS485 port already used by another app.

All data received from system serial interfaces with the help of RS485 Protocol Analyzer can be recorded to a file and copied to the clipboard so that a developer is able to analyze it at a later time.

The solution comes in handy when developing an application or a driver, testing and optimizing COM-based devices.

How to sniff RS485 data


Now let’s consider the easiest and most efficient way to sniff RS-485 data with RS485, RS422 monitor software.

  1. Download RS485 Analyzer and install it on your machine. Start the app.
  2. Start a new monitoring session: on the Main menu, select Session>New session.
  3. Choose the desired mode for data display. (Sniffed data can be shown to you in five different views: Table, Line, Dump, Terminal, and Modbus. You can enable one, several, or all the visualizers to work at a time.)
  4. In ‘Capture options’, select the activity to monitor – ‘Create/Close, Read/Write, Device Control’.
  5. Finally, click "Start monitoring".

What is RS485 Modbus protocol?

RS485 Modbus protocol

One of the main features that differs RS485 communication from any other serial communication is the format of data exchanged. While RS232 devices connect over text (ASCII) protocols, most RS485 devices use Modbus.

Modbus is a serial communications protocol that is widely used by industrial electronic devices. In Modbus, the connection is established between a master (host) and slaves (COM-based devices). Modbus helps access the configuration of the devices and read the measures.

The data exchange is initiated by a host. The host can switch its RS-485 driver to the transmission mode on its own, while the other RS485 drivers (slaves) work in the receiving mode. In order for a slave to answer the host over the communication line, the 'master' sends it a special command, which gives the intended device the right to switch its driver into a transmission mode for a certain time.

RS485 Modbus data logger


The dedicated solution, Modbus Sniffer, allows reading and recording Modbus data transferred over RS485/RS422/RS232 ports. The software provides full support for Modbus RTU and Modbus ASCII protocols. The unique functionality of the app allows logging and filtering Modbus data communicated by specialized serial devices.

Serial Port Monitor

Requirements: Windows XP/2003/2008/Vista/7/8/10/Server 2012 , 9.16MB size
Version 7.0.342 (16th Jan, 2018) Release notes
Category: Serial Port Software