What is RS485 and how is it used in industrial control systems?

Nov 23 , 2021
Today you will learn about RS485. if you are familiar with RS232, then you probably already have some understanding of it. If not, check out our other blog post, What is RS232?

Just like its older brother RS232, RS485 is a form of serial communication. Think of RS485 as the younger, faster brother of RS232.

The reason it is such an improvement over RS232 is that it not only allows transmission between a single device, but also allows multiple devices to be connected at once via the communication bus.

The configuration and specifications of RS485 make it faster and extend the range of data transfer.

You are not limited to 50 feet of cable as you are with RS232. The maximum length of RS485 cable is typically listed as 1200 meters or about 4000 feet.

Like RS232, RS485 does not have an actual standard connector. Many times the old DB-9 connector is used.

In some applications, a terminal block is used instead of a connector. This has helped RS485 to become a widely used and very versatile form of serial communication.

Another advantage over RS232, which I mentioned earlier, is the number of devices that can be connected. RS485 can handle up to 32 connected devices. Whereas RS232 can only handle one. This allows users of production facilities to connect larger applications and entire machines with a single protocol.

Another advantage of RS485 is that it is not susceptible to noise problems. Electrical noise can cause all kinds of problems for electronics.

The RS232 references the ground to interpret the data it is transmitting. This allows electrical noise to cause interference.

RS485 avoids this problem and also allows the cable to be shielded against noise. You may be wondering what I can do with serial communications, specifically RS485.

A very common example in automation is the remote control of a VFD or variable frequency drive. Many people just call them motor drives.

A simple network of PLCs, VFDs and HMIs allows remote control of motors in an industrial environment.

Let's say you're monitoring a pump that fills a container with water. You decide that the water is not filling fast enough to keep up with demand. Through your RS485 network, you can monitor and control that pump remotely from your control center.

This is possible because the HMI is connected to your PLC via RS485. the PLC is also connected to the VFD via RS485.

The PLC is programmed to monitor the water level in the vessel. It can also perform manual flow control if needed.

All of this allows you to speed up the VFD that controls the speed of that pump. Thanks to the RS485 communication, the problem is solved.
Let's talk about what we now know about RS485. It is faster and has more features than the old RS232.

Users can use longer cables, up to 1200 meters or 4000 feet.

The great thing about RS485 is that you can connect up to 32 devices on a single system. Whereas RS232 can only handle one at a time.

There is no standard connector for this protocol. Most manufacturers still use DB-9 connectors.

You can also still simply use a terminal block. This makes utilizing RS485 simple. All in all, RS485 is truly fully automated with its ability to connect so many devices at once.

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