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RS485 vs Modbus: what are the main differences

Olga Weis Olga Weis Sep 27, 2021

In the modern world of internet and network communication, a plethora of tiny but critical details exist that help provide everyone with limitless information at their fingertips.

Many things must work together to make even the most basic search request possible- whether that be the client-side network type, or the internet protocols to databases in use.

One specific element that contributes to a smooth and effective information exchange is the MODBUS, and RS485 protocols.

Often incurring much confusion for many people, RS485/MODBUS protocols provide two unique (but related) concepts. In this article, we’ll cover what readers need to know about these protocols, why they’re important, and how to use them.

Modbus vs RS485

Slave communication in MODBUS

Being that RS485 and RS232 aren’t directly compatible, it isn’t necessary to use the correct interfaces to ensure successful signal transmission. While users can utilize RS232 to RS485 gateways, it’s much more commonplace for users to directly travel from RS485 to USB, GSM, or Ethernet.

This method is less costly, as well as avoids the necessity for extra components.

Data transmitted via an RS485 interface usually employs the MODBUS protocol. Alternatively, an RS232 device uses text (ASCII) protocols. The main difference is that Modbus articulates the protocol type, whereas RS485 defines the protocol's signal level.

When using an RS485 communication device, the aforementioned distinction means that users should take some time to understand a bit about MODBUS protocol.

A Brief MODBUS Tutorial

The MODBUS RS485 protocol articulates communication between hosts (AKA “Masters”) and devices (AKA: “Slaves”), allowing a query for device monitoring and configuration.

Messages transmitted by MODBUS provide basic read and write operations via binary registries (known as “Coils”), and 16-bit words. Slave devices respond solely to host/master requests. The master/host always initiates any communication.

If users have multiple devices connected on the RS485 bus (in parallel), each individual device needs a specific MODBUS Slave ID.

Every MODBUS request starts with the host contacting the Slave ID of the desired device, and the answer replies with the Slave ID of the transmitting slave device.

Thus, Modbus protocols literally define the messaging structure utilized while exchanging data between the master and the slave(s) (or devices.)

However, it should never be confused with a medium of communication. MODBUS only forms the messaging structure, but is not the physical medium of data transmission.

rs-485 signal line

In most common instances of industrial data exchange (or communication during process automation), a BAS master is usually involved. BAS (Building Automation System) is a communication gateway device and PLC, or it’s the software app running on a computer.

In order for the data exchange to happen, the host requires a medium that not only facilitates the exchange but also determines the pace.

How To Monitor And Analyze MODBUS Data

Modbus Test Software is a sniffer tool that allows you to analyze RS232/RS422/RS485 interfaces transmitting MODBUS communications. SPM’s excellent functionality enables easy detection and resolution of problems encountered when engaged in MODBUS testing and debugging. A distinctive feature of this tool is how it can display and log all data flowing through your system’s COM port.

Using this MODBUS sniffer software’s advanced search and filter options, you can display only the relevant subset of data that you desire. SPM also contains a built-in terminal for executing text commands. This user-friendly application supports exporting data in various formats and has many customizable options.

Modbus Tester Software

Modbus Analyzer is focused on logging, debugging and displaying Modbus RTU and ASCII serial data transmitted over system’s RS485 ports.
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This professional Modbus test software allows you to:

  • Connect to and monitor serial ports already in use by another program. Real-time display of all data received by the port is available for immediate monitoring and debugging.
  • Monitor multiple serial ports simultaneously. You can synchronously analyze the activity of several serial ports in a single session. Data flowing in both directions is presented on a first-in first-out basis in a single log for ease of viewing.
  • Your data can be viewed in a number of formats including table, terminal, line, and dump views. Multiple views can be seen at the same time and you can use filters to fine-tune your monitoring.
  • Perform emulation of data transferred to a serial device to monitor activity when exposed to specific commands.

Frequently Asked Questions

No, MODBUS and RS485 are not the same. The reason being that both protocols are related concepts, and work together to successfully function.
Two variations of the MODBUS protocol exist:

In order to speak with a MODBUS device, users must always employ the same mode configured within the device. All devices that genuinely follow the standard will support the MODBUS RTU mode.

In reality, it’s always the MODBUS RTU mode being used. This is mainly because MODBUS ASCII offers no advantage since all messages are, in any instance, challenging to code by hand.
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