Modbus: An Introduction to the Protocol

When you’re running a modern factory, you have many machines that need to communicate with each other. These conversations occur over communication protocols that integrate with whatever native ‘language’ the machines speak. However, communications must translate each language to allow other networked devices to understand them. Modbus is one of the protocols that allow this communication to occur.

What is Modbus?

This is the oldest and most popular of these communication protocols. Published by Modicon in 1979, it was originally used with the company’s programmable logic controllers or PLCs. But Modicon developed Modbus as an open protocol. Anyone could use them for free without licensing. Since its original development many software vendors, manufacturers, and other groups/organizations have supported the protocol.

The communication protocol uses serial lines to send data between devices. This is as simple as a single serial cable connected to serial ports on a Master device and a Slave device. Data moves at 9600 baud (bits per second) as series of zeroes and ones. Each zero or one is a single bit. Data follows regular patterns so 8 bits identifies as a larger byte.

Modbus is a type of SCADA (Supervisory Control and Data Acquisition) automation protocol. It is now owned by Schneider Electric. It is used to help devices and equipment communicate by providing a common language all can understand. This allows for different nodes on the network to interact with request/response type messages.

What Are Modbus Communication Protocol?

The original Modbus interface used serial RS-232 communication. However, as the technology developed, options expanded to include serial RS-485, serial RS-422, and Ethernet. Formatted Ethernet packets embed Modbus messages inside, creating versatile setups. Additionally, networks designed with mixed drops can run different protocols altogether. For example, a single network could run three drops, one using MB Ethernet TCP/IP, another MB RS-232, and a third MB RS-485.

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What is a Programmable Logic Controller?

A useful Infographic covering some of the most-asked questions about PLCs.

FAQs, and an Infographic you can use.

How does a programmable logic controller work?

A PLC works like a computer. The CPU manipulates and processes input signals and executes instructions to control output devices.  These are the controllers behind most modern industrial automation.

What are programmable logic controllers used for?

PLCs are used in industrial settings for the control of light fixtures, machinery, and assembly lines. They are also used to control other things such as amusement park rides. 

Programmable Logic Controller Infographic
Download a high-quality PDF of this infographic below.

Are there different types of PLCs?

Yes. There are Relay Output PLCs, Transistor Output PLCs, and Triac Output PLCs.  The name of each type of PLC derives from its output. Relay outputs are solid-state outputs or mechanical contacts. Transistor outputs are solid-state switches, but can only handle DC current. Low-power AC loads use Triac outputs. This includes motor starters and lighting.

Mechanical design is another way to categorize PLCs. For example, a “brick” is one type of PLC design. This is a small PLC that includes the CPU, the power supply, and the inputs/outputs in one small case. One type of brick PLC is the DirectLogic 05.

An example of a Brick PLC.
A Direct Logic D0-05DA is a type of Brick PLC.

Other PLCs are modular. Modular PLCs have a rack system that has space for different modules. These modules can provide different functions and may act as the PLC’s power supply, I/O, communication interface, or processor.

An example of a modular PLC
The Automation Direct DirectLogic 405 is an example of a modular PLC.

How are PLCs programmed?

PLCs use a traditional programming language that mimics circuit diagrams; this is called ladder logic. Other PLCs may use programming languages like Function Block Diagram (FBD), Instruction List (IL), Sequential Function Chart (SFC), or Structured Text (ST). IEC 61131–3 standard defines these languages.  

What are the advantages of using a PLC?

There are several. PLCs are easy to install, maintain, and troubleshoot. They are flexible by nature and offer many different operations. Also, they offer a rugged design specifically for industrial processes. Finally, they’re cost-effective. 

Are There Disadvantages to PLCs?

The most common disadvantage to PLCs stems from interoperability or lack thereof.

Originally, manufacturers created PLCs using their own proprietary software and programming. When machines did their own thing, separated from the rest of the machinery on the factory floor, that was fine.

But now we have IIoT efficiencies to think about. Machines need to “talk” to each other and share information. Unfortunately, older PLC design doesn’t allow this.

Luckily, industry groups are now working to solve this problem through new forms of open-access middleware like MQTT, AMQP, and CoAP.

If you need a PLC for your industrial processes, talk to the staff at AX Control today. We’re here to help! We offer many brands of PLC systems and replacement parts, including the popular GE Fanuc PLCs.

Koyo Direct Logic 205: Most Common FAQs

Koyo Direct Logic 205 D2-04B
The Koyo DirectLogic DL205 4 Slot AC Base

Here at AX Control, we field a lot of questions about the devices we sell. This is the first in a series of posts that will answer some of those FAQs. This post will focus on the Koyo Direct Logic 205 series.

What is the Koyo Direct Logic 205?

Automation Direct DirectLOGIC 205 controller is a micro-modular programmable logic controller. These PLCs have a built-in power supply and come in 3-slot, 4-slot 6-slot, and 9-slot versions. Certain versions of the DL205 support up to four expansion bases.

How can I load programs onto a DirectLogic 205 PLC?

Downloading a program and connecting to the Koyo DirectLogic PLC is covered extensively in the AutomationDirect manual. Here are the basic steps:

  • Start the Linking Process by connecting via the PLC menu bar(first option in the dropdown menu.)
  • Add a new link by clicking “Add to” in the ‘select link’ pop up menu. If you have created previous communications links the menu will give you the option to select which link you want to proceed with.
  • Select a COM port. You should have previously connected a communications cable to your desired COM port. Now re-enter that COM port location into the pop-up menu and hit “Next.”
  • Select a PLC family from the pop-up menu. In this case, you will choose 105/205/405 family.
  • Select your communications protocol. Leave the PLC address set to the default.
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