RS485 communication guide

What is RS-485?

RS-485 (currently known as EIA/TIA-485) is a standard interface of the physical layer of communication, a signal transmission method, the 1st level of the OSI (Open System Interconnection) model. RS-485 has been created in order to expand the physical capabilities of RS-232 interface.

The serial EIA-485 connection is done using a cable of two or three wires: a data wire, a wire with inverted data, and, often, a zero wire (ground, 0 V). This way, transmitters and receivers exchange data via a twisted-pair cable of 22 or 24 AWG solid wires.

The main idea here is to transport one signal over two wires. While one wire transmits the original signal, the other one transports its inverse copy. Such transmission method provides high resistance to common-mode interference. The twisted-pair cable that serves as a transmission line can be shielded or unshielded.

RS-485 communication: main features

Despite the wide variety of modern alternative solutions, today RS-485 technology remains the basis of many communication networks. The major advantages of RS-485 interface are:

• Two-way data exchange via one twisted pair of wires;
• support for several transceivers connected to the same line, i.e., the ability to create a network;
• long length of the communication line;
• high transmission speed.

Now, let’s take a closer look at the main characteristics of RS-485 communication:

Bi-directional half-duplex data transmission

Serial data stream can be transported in one direction, data transfer to the other side requires using a transceiver. A transceiver (commonly referred to as 'driver') is a device or an electrical circuit that forms a physical signal on the transmitter side.

Symmetrical communication channel

Receiving or transmitting data requires two equivalent signal wires. The wires are used to exchange data in both directions (alternatively). With the help of a twisted pair cable, the symmetrical channel significantly increases the stability of a signal and suppresses electromagnetic radiation generated by the useful signal.

Multi-pointing

RS-485 communication line can work with multiple receivers and transceivers connected. At the same time, one transmitter and several receivers can be attached to one communication line at a time. All the other transmitters that need to be connected should wait until the communication line is free for data transmission.

What is RS485 communication protocol?

When RS485 communication line is ready for operation at the physical level, it's time to think about the data transfer protocol - an agreement between the system's devices on the format of the data packets transmission.

By the nature of RS-485 interface, RS-485 devices cannot transmit and receive data at the same time, as it leads to a conflict of transmitters. Therefore, the deterministic behavior is mandatory to avoid collisions of data packets.

In RS485 communication protocol, the commands are sent by the node which is defined as a master. All other nodes connected to the master receive the data over RS485 ports. Depending on the information sent, zero or more nodes on the line respond to the master.

With that said, this type of communication is not the only possible way to exchange data over RS485 protocol. There are some other implementations of RS485 networks where every node can start the data transfer on its own. However, the risk of possible data loss is higher in such networks.

Modbus is one of the simplest protocols for the interaction of devices with each other. It is at the same time easy to implement for equipment manufacturers, which is the primary reason for its prevalence, and at the same time, it is difficult for an engineer, programmer, because it shifts onto his shoulders all the difficulties of implementation in the final solution, requiring him to work with multi-page tables of registers and variables, their addresses, various functions of writing and reading and data conversion.