Home A - Z
Page Index

RS-485 (Recommended Standard 485)

What is RS-485 and why should we care about it?

RS-485 is a standard which defines the electrical characteristics of drivers and receivers for serial communications systems. RS-485 is commonly found in industrial & security systems as it allows multiple devices to be connected together over long distances with very simple wiring. Unfortunately, even though RS-485 is wide spread throughout the industry, there's often an observable lack of understanding as to how it works and how it should be connected which we will try to address below.

History of RS-485

The standard as we know it was initially introduced back in 1983 by the Electronic Industries Alliance (EIA) back when the standard was first introduced the EIA labelled all of its standards with RS (Recommended Standard). As of today, the EIA has disbanded and the standard is now maintained by the Telecommunications Industry Association (TIA) as TIA-485, however it's original designation of RS-485 is still widely preferred by manufacturers and engineers alike.

What is serial communication?

Put simply, serial communication is a method of sending data between multiple devices by transmitting bits one at a time utilising a single wire or channel.

How it works

  • Data is sent in a packet or "frame" of bits.
  • The receiver interprets the voltage on its wire as a bit of data.
  • A high voltage is interpreted as 1 and a low voltage is interpreted as 0.
  • The receiver interprets the bits of data over time to get a message from the sender.

For more information on Serial Communication please see our explanation page: Communications/Serial

Full or Half duplex?

Well firstly, what do we mean by duplex?

A duplex communication system refers to any point-to-point (PTP) system composed of two or more connected devices which can communicate with each other in both directions. There are two common duplex communication systems: Full-Duplex (FDX) and Half-Duplex (HDX).

In a Full-Duplex system, communication can occur in both directions simultaneously. Old-style telephone service lines are a prime example of an FDX communication system; both parties are able to speak and be heard by the other party simultaneously, there is a constant channel each way between them.

In a Half-Duplex (also known as a semiduplex) communication can only occur one way at a time. If device A would like to send data to device B, but device B is already sending data to device A, then device A will have to wait until device B has stopped transmitting to send data.

When it comes to RS-485, the total line count defines whether the system is capable of half or full duplex. In a half duplex you only need 2 lines or 1 channel (DATA+ & DATA-) duplex RS-485 line you should have 4 lines or 2 channels (DATA TX+, DATA TX-, DATA RX+, DATA TX-).

Common Ground

One big issue that I have often seen in access control system installations which utilise the RS-485 standard for communication is no common ground wire being carried the full length of the data line. There's a lot of contradicting information online as to whether or not a common ground is required. To put it simply, you can happily run RS-485 data lines with no common ground over small distances for two reasons:

  1. You've provided a common ground via localised power supplies, in this case the localised power supplies have very little difference in their earth readings and therefore act as an indirect common ground.
  2. You haven't connected any ground at all, in which case the transceiver chip is creating a "virtual" ground path via semi-conductors that make up the transceiver chip.

In instance #2, where no ground is connected the RS-485 transceiver will function with its "virtual ground" provided the common mode voltage is little more than a few volts. If the common mode voltage gets too large, it will limit the transceiver's ability to create a virtual ground. Whilst all transceiver chips have their own specified common mode voltages, it is best practice to assume this is zero.

Common Mode Voltage

The term common mode voltage is a refence to the voltage potential between the grounds of two different devices/systems. The "ground" is highly unlikely to actually be at exactly zero volts this is beyond the scope of this explanation however here is a YouTube video by Vocademy which elaborates on this in detail. The key ones that commonly cause issues on data lines are magnetic fields and line loss.

What happens if there is no common ground?

The lack of a common ground will cause devices on the RS-485 line to mis-read bits of data. There is no definitive answer to what issue occurs as it wholly depends on how incorrectly the data is being read. Most commonly devices will simply refuse to communicate.

So is RS-485 actually a 2 wire system?

Basically, yes it is. 2-wire system is NOT the same thing as a 2 conductor system. 2-Wire & 4-Wire are solely a reference to how many data lines there are. Full duplex required 4 data lines hence 4-Wire whereas half duplex required 2 data lines hence the term 2-Wire.