## What is RS-485 and why should we care about it?
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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.
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## What is RS-485 and why is it important?
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RS-485 is a standard for serial communication that defines the electrical characteristics of drivers and receivers. It is widely used in **industrial, fire, and security systems** due to its ability to support multiple devices over long distances with simple wiring. Unlike RS-232, RS-485 uses **differential signaling**, which improves noise immunity and allows for longer cable runs.
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Despite its widespread use, there is often a lack of understanding regarding how RS-485 should be correctly implemented. This guide will address its key features, wiring methods, and considerations for ensuring reliable operation.
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## History of RS-485
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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.
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RS-485 was introduced in **1983** by the **Electronic Industries Alliance (EIA)** as an improvement over earlier serial standards. Initially labeled with the "RS" (Recommended Standard) prefix, the EIA has since been disbanded, and the standard is now maintained by the **Telecommunications Industry Association (TIA)** as **TIA-485**. However, the term "RS-485" remains the most commonly used designation in the industry.
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## What is serial communication?
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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.
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## How RS-485 Works
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**How it works**
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- Data is sent in a packet or "frame" of bits.
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- The receiver interprets the voltage on its wire as a bit of data.
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- A high voltage is interpreted as 1, and a low voltage is interpreted as 0.
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- The receiver interprets the bits of data over time to get a message from the sender.
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RS-485 transmits data **one bit at a time (serially)** over a communication channel. It supports **both half-duplex and full-duplex communication** and is designed for multi-device networks, unlike RS-232, which only supports point-to-point connections.
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For more information on Serial Communication, please see our explanation page: [[Communications/Serial]]
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**Key characteristics of RS-485:**
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- **Differential signaling** → Uses two wires per data line (DATA+ and DATA-), reducing noise interference.
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- **Multi-device capability** → Supports up to **32 devices** on a single bus (expandable with repeaters).
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- **Half-duplex and full-duplex support** → Can use **two** wires (half-duplex) or **four** wires (full-duplex).
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- **Long-distance transmission** → Supports distances up to **1200 meters (4000 feet)**.
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- **Higher noise immunity** → Ideal for industrial environments with electrical interference.
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## Full or Half Duplex?
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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-).
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### **What do we mean by duplex?**
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Duplex communication refers to the ability of a system to send and receive data. RS-485 supports both **half-duplex** and **full-duplex** modes.
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### **What’s the difference?**
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- **Half-Duplex (2-wire RS-485)** → Communication occurs in only one direction at a time using a single pair of wires (DATA+ and DATA-).
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- **Full-Duplex (4-wire RS-485)** → Separate transmit (TX) and receive (RX) lines allow data to be sent and received simultaneously.
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For further information on what half and full duplex refer to, please see [[Communications/Serial]].
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Most RS-485 implementations use **half-duplex**, as it requires fewer wires and is sufficient for most industrial applications.
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## Common Ground
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## Common Ground Considerations
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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:
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One major issue in **RS-485 installations** is whether or not a **common ground** is required. There is conflicting information online, but in general:
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1. 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.
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2. 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.
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1. **A common ground is not always necessary for short distances** → If all devices share a common power source, an additional ground wire may not be needed.
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2. **For longer distances, a common ground is recommended** → Variations in ground potential between devices can lead to communication failures.
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3. **If no ground is provided, the transceiver may create a virtual ground** → This works only if the **common mode voltage** stays within safe limits.
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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.
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### **Common Mode Voltage**
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The term **common mode voltage** refers to the voltage difference between the grounds of two different devices. Ground loops, electromagnetic interference, and power supply variations can introduce unwanted voltage offsets.
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If the common mode voltage exceeds the specified limits of the transceiver, communication errors can occur. For a detailed explanation, watch this [YouTube video](https://www.youtube.com/watch?v=0GzEt2Sfe4k) by Vocademy.
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### **What happens if there is no common ground?**
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Without a common ground, RS-485 devices may **misread data bits** or fail to communicate altogether. The severity of issues depends on factors such as cable length, grounding differences, and environmental interference.
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#### Common Mode Voltage
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## RS-485 Connector Types & Pin Configuration
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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](https://www.youtube.com/watch?v=0GzEt2Sfe4k) 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.
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RS-485 does not have a standardized connector like RS-232 (which often uses DB9). Instead, it commonly uses **terminal block connectors** or **RJ45 connectors** in some applications.
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.
| Max Devices | Up to 32 (or more with repeaters) | 1-to-1 |
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| Noise Immunity | High | Low |
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| Data Rate | Up to 10 Mbps | Up to 115.2 kbps |
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| Duplex Mode | Half or Full-Duplex | Full-Duplex |
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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).
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RS-485 is a versatile and robust serial communication standard widely used in **industrial automation, fire and security systems, and embedded electronics**. While it requires proper wiring, termination, and grounding considerations, its ability to communicate over long distances and support multiple devices makes it **superior to RS-232** for many applications.