Tag Archives: RS-485

How are RS232 and RS485 Different?

When engineers need to connect electronic equipment, they resort to serial interfaces such as the RS-232 and RS-485. Although dozens of other serial data interfaces exist today, most are meant for use in specific applications. A few of them are considered universal, such as I2S, MOST, FLEX, SPI, LIN, CAN and I2C. Other high-speed serial interfaces are also used, including Thunderbolt, HDMI, FireWire, USB, and Ethernet. Despite the proliferation of interfaces, the two legacy interfaces, RS-232 and RS-485, continue to survive, used in several applications.

As a rule, serial interfaces provide a single path for data to be transmitted over a cable or wirelessly. Although some applications do use parallel buses, serial interface alone provides the only practical option for high-speed data movement today over any distance greater than several feet.

RS-232

RS-232 is one of the oldest serial interfaces, originally established in 1962, as a method of connecting a DTE or data terminal equipment such as a teletypewriter to a DCE or data communications equipment. Personal computers earlier had an RS-232 port, commonly called the serial port, to connect to a printer or other peripheral device. Embedded computer development systems still use the port today, as do many scientific instruments, and several industrial control equipment.

Officially, the standard defining the RS-232 serial interface is the EIA/TIA-232-F, with F signifying the most recent update. According to the standard, a logic 1 is defined as a voltage between -3 and -25 V, and a logic 0 as a voltage between +3 and +25 V. The logic 1 is generally termed as a mark, with logic 0 being termed as a space. Any voltage between +3 and -3 V is termed invalid and is rejected, providing a huge noise margin for the interface. The configurations of the receiver and transmitter are both single-ended and referenced to ground or 0 V.

The cable medium in RS-232 can be simple wires in parallel or a twisted pair. According to the standard, the cable length must not exceed 50 feet. However, by reducing the data rate, it is possible to use longer lengths of cable. For a 50-foot cable, the highest data rates in RS-232 are roughly 20 Kbits/s, and matched generator and load impedances are necessary for eliminating reflections and data corruption. Although earlier 25-pin connectors were used, the de-facto standard for RS-232 is the 9-pin DE-9 connector today.

RS-485

The EIA/TIA standards also define the RS-485 interface, now commonly known as TIA-485. This is not only a single device-device interface, but is a complete communication bus used for simple networking of multiple devices.

Rather than a single-ended voltage referenced to the ground, the RS-485 uses differential signaling on two lines. A logic 1 is a voltage level greater than 200 mV, while the logic 0 is a level greater than +200 mV. The maximum cable length for RS–485 is about 4000 feet or 1200 m, with typical data rates as 100 Kbits/s. However, compared to the speed of the RS-232 interface, a 20-meter cable in RS-485 can allow a maximum data rate of 5Mbits/s. Industrial control equipment using the RS-485 use the 9-pin DE-9 connector.

RS-485 – The Wired Communication Standard

TIA/EIA-485 is a popular wired communication standard published by the TIA/EIA or the Telecommunications Industry Association/Electronics Industries Association. This standard is also known as the RS-485 and uses differential signaling enables the standard to transmit data over long distances for factory automation and in noisy industrial environments.

This is because differential signaling allows rejection of common mode noise, while the twisted pair cable ensures the most received interference comes as common mode. When used over long distances, the standards improve the chances for ground potential differences, while the wide CMR or common mode range of the standard ensures that the network operates satisfactorily, even when there are large common mode voltages present.

In practice, both the transmitter and the receiver have non-inverting and inverting pins. Bidirectional communication over a single cable can use half-duplex devices, where the corresponding Receiver and Transmitter terminals connect to the same IC pins. Networks can also use two cables for bidirectional communication, and employing full-duplex devices, only, the Receiver and Transmitter terminals now must connect to separate pins.

The number of transceiver models available in the market is huge, and that makes it a challenge picking out the best and most cost-effective device for a specific application. That requires considering the common design considerations, examining the electrostatic discharge (ESD) protection and comparing the Human Body Model. Other important points to be considered are the over voltage protection (OVP) and data skew in case of high-speed transmissions.

Requirements of RS-485

Although the published standard for the RS-485 is over 14 pages long, the most important requirements are:

The differential output voltage generated must be over ±1.5V, while the receiver must be capable of detecting signals with a minimum of ±200mV. This combination makes sure the devices can tolerate attenuation from long cables and there is a robust noise margin available.

As the standard allows multiple drivers on the bus, each transmitter must have an enable pin giving it tri-state output capability. This ensures true bidirectional transmission over a single cable.

Transmitters must have high output current capability to drive long cables and cables with double termination, especially for high-speed bidirectional transmission.

The CMR should be at least -7V to +12V. This allows using RS-485 over networks of 1220 m or 4000 feet. Long distances can involve ground potential differences and a high CMR helps to tolerate them in noisy environments. Additionally, devices with different supply voltages can also communicate on the same bus because of a large CMR.

The receiver input resistance must be over 12KΩ. According to the standard, there can be 32 devices on a bus.

The Basic RS-485 Transceiver

People often use less expensive RS-485 transceivers for simple, short, low-node-count networks. This works because short networks do not involve much CMV or common mode voltage and OVP, and they can work with the CMR specified by the standard.

When there are less than 32 modes in the network, fractional unit load devices are not necessary. Moreover, when cables are not frequently connected and disconnected, ESD protection is also not necessary. However, most basic devices now include the ±8 to ±15 KV Human Body Model for ESD protection.