RS-485 (Recommended Standard 485)

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

RS-485 is a standard that defines the electrical characteristics of drivers and receivers for serial communications systems. RS-485 is commonly found in industrial, fire and security systems as it allows multiple devices to be connected over long distances with very simple wiring. Unfortunately, even though RS-485 is widespread throughout the industry, there's often an observable lack of understanding of 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 in 1983 by the Electronic Industries Alliance (EIA). When first introduced, the EIA labeled all of its standards with RS (Recommended Standard). As of today, the EIA has been disbanded, and the standard is now maintained by the Telecommunications Industry Association (TIA) as TIA-485. However, its 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 using 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?

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 system, only two lines or one channel (DATA+ and DATA-) are needed. In a full-duplex RS-485 system, there should be four lines or two channels (DATA TX+, DATA TX-, DATA RX+, DATA RX-).

For further information on what half and full duplex refer to, please see Communications/Serial.

Common Ground

One big issue often seen in access control system installations that utilize the RS-485 standard for communication is the absence of a common ground wire 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 successfully run RS-485 data lines without a common ground over small distances for two reasons:

You've provided a common ground via localized power supplies. In this case, the localized power supplies have very little difference in their earth readings and therefore act as an indirect common ground.
You haven't connected any ground at all, in which case the transceiver chip is creating a "virtual" ground path via semiconductors 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. While 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 reference 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 that elaborates on this in detail. The key factors 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 misread 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. A 2-wire system is NOT the same as a 2-conductor system. 2-wire and 4-wire are solely references to how many data lines there are. Full duplex requires four data lines (hence 4-wire), whereas half duplex requires two data lines (hence 2-wire).