RS485 communication guide

How far can you run RS485?

Communication network built on the RS-485 interface consists of transceivers connected by a twisted pair (two twisted wires). The basic principle of RS-485 interface is differential (balanced) data transmission. That means one signal is transported over two wires. With that, one wire of the pair transmits the original signal and the other one transports its inverse copy.

As a result of differential signal transmission there is always a potential difference between the wires. This ensures high resistance to common mode interference. In addition, the twisted pair may be shielded, which ensures the protection of transmitted data. All this allows sending data over long distances at relatively high speeds, which can reach 100 kbits/s at 4000 feet.

4000 feet or about 1200 meters is the maximum cable length in RS-485 communications. A general guideline, however, is that the product of line length (in meters) and the data rate (in bits per second) should not be more than 10^8. For instance, a 20-meter cable allows a maximum data rate of 5 Mbits/s.

What is RS485 Modbus protocol?

One of the main features that differs RS485 communication from any other serial communication is the format of data exchanged. While RS232 devices connect over text (ASCII) protocols, most RS485 devices use Modbus.

Modbus is a serial communications protocol that is widely used by industrial electronic devices. In Modbus, the connection is established between a master (host) and slaves (COM-based devices)*. Modbus helps access the configuration of the devices and read the measures.

* On July 9, 2020 Modbus Organization replaces Master-Slave with Client-Server to describe Modbus communications, characterized by communication between client device (s), which initiates communication and makes requests of server device(s), which process requests and return an appropriate response (or error message).

The data exchange is initiated by a host. The host can switch its RS-485 driver to the transmission mode on its own, while the other RS485 drivers (slaves) work in the receiving mode. In order for a slave to answer the host over the communication line, the 'master' sends it a special command, which gives the intended device the right to switch its driver into a transmission mode for a certain time.

RS485 vs RS232: Comparing serial communication protocols

The RS485 and RS232 serial communication protocols have been in use for over 50 years and are still widely used throughout business and industry. Expanding the capabilities of the RS232 standard was the impetus for developing the RS485 protocol. The following table provides an overview of the two standards.

Now, let’s take a look at the main differences between the two protocols.

Number of Transmitters and Receivers

The RS232 Serial Interface is designed to connect two devices. The protocol enables communication between a single transmitter and receiver. Using an RS485 interface, up to 32 serial devices can be connected to one transmitter.

Making effective use of serial devices often involves converting between RS232, RS485, and USB signals. This is partly due to hardware manufacturers’ concentration on providing USB connectivity rather than serial ports on laptop and desktop machines. Conversion enables older devices to coexist with new hardware that lacks serial interfaces.

Operational Distance

Devices using the RS232 protocol are limited to a distance of 15 meters between transmitter and receiver while still achieving the maximum data transmission rate. This length can be extended if slower data rates can be tolerated.

The operation distance is substantially extended in the RS485 protocol. It supports maximum data rates at a length of 1200 meters, making the RS485 protocol an excellent choice for communication between physically distant pieces of equipment.

Data Transmission Speed

The transmission speed possible with an RS232 interface is 1Mb/s for a distance of up to 15 meters. RS485 offers greater speeds of up to 10Mb/s over a distance of 15 meters. When taken to its maximum length of 1200 meters, the RS485 protocol transmits data at 100Kb/s.

Electrical Noise Concerns and Ground Potential

An RS232 interface is a system based on voltage levels and performs optimally in situations where there are minimal differences in ground potential. Environments that exhibit high levels of electrical noise and variable ground potential impact RS232’s ability to transfer data efficiently and can lead to data loss or corruption.

The RS485 protocol employs a differential voltage system that allows it to operate effectively in environments with higher levels of electrical noise. A byproduct of the differential voltage system is the extended data transfer distance, increased transmission speed, and lower voltage use seen with RS485.