RS-485 Serial Communication FAQ

What is RS-485 Serial Communication?

RS-485 serial communication is an industry standard communications method for connecting multiple devices on a communications network bus. It is implemented by many, many different devices, is very reliable and well understood.

What hardware do I need for RS-485 serial communication?

Usually you need one an RS-485 serial interface, which installs in your computer and presents the RS-485 serial network as a serial port (such as a COM port on a PC). The actual wiring can be implemented with good quality communications cable, such as one or more pairs on a CAT5 ethernet cable.

Here are a couple RS-485 serial interfaces we have used:

What software do I need for RS-485 serial communication?

RS-485 communication can be implemented in most programming languages by sending data to the serial port on the computer assigned to the RS-485 serial network. National Instruments LabVIEW™ fully supports RS-485 serial communications.

What is the difference between RS-485 serial and Modbus™?

Strictly speaking, RS-485 (or formally EIA-485) is an electrical standard, but more commonly, RS-485 serial communication refers to using the electrical standard to send data between devices on a serial network bus, where each device can be separately addressed and where all the devices communicate using a given transmission format.

Modbus™ is a specific standard for a transmission format that may (or may not) be implemented by the devices on the serial network bus, depending on the device manufacturers’ implementation of RS-485 serial communication. National Instruments has an article on this topic here

What is the difference between a 2-wire and 4-wire RS-485 network?

In a 4-wire RS-485 network, all devices have dedicated wiring pairs for both transmitting and receiving (full duplex). This means that a device can be listening and transmitting at the same time. In a 2-wire RS-485 network, a single pair of wires functions for both transmitting and receiving (half duplex). This means that a device can either be transmitting or listening, but not both at the same time.

Isn’t RS-485/modbus dead or old fashioned in the age of Ethernet and other advanced protocols?

Hardly. It isn’t the newest thing on the block. Or the fastest. However, literally hundreds of thousands of process control applications use RS-485 every day for SCADA applications. Lots of devices such as programmable logic controllers (PLCs), single loop controllers (SLCs), temperature and process controllers, transmitters, transducers, and control valve positioners speak RS-485 out of the box. It is a very well-known, well-understood and robust communications protocol that has been around and will continue to be around for some time. You may or may not build something from the ground up using it, but for existing RS-485 processes, it can be a lifesaver.

Where can I get more information on wiring an RS-485 serial communications network?

Here are some links to reference information wiring:

What is the difference between RS-232, RS-422 and RS-485 serial communications?

National Instruments has excellent articles on this topic here and here.

Can I connect an RS-485 serial bus to the RS-232 serial port on my computer?

There are several RS-232 to RS-485 converters available from different manufacturers.

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