Zachariah Peterson
|  Created: August 24, 2017  |  Updated: September 7, 2022
In the world of electrical design, RS-485 communication offers robustness and resiliency for cable-based systems. However, in today's world of high-speed serial protocols like USB, and with just about everything having a wireless connection, you'd probably wonder whether RS-485 is still a relevant skill area for today's PCB design professionals.As wireless communication technology becomes more ubiquitous with each passing day, and as the frequencies get higher, it's appropriate to revisit this type of interface to see if it will still be relevant in your designs.
RS-485 and Its Applications
The RS-485 interface is a 2-wire half-duplex interface which must be balanced. An additional two wires can be used to implement a 4-wire full-duplex interface. It has a few important specifications regarding the cable and interface specification:
Specification | Value |
Maximum Data Rate | 10 Mbps |
Cable length | 4000 feet (1219 meters) |
TX/RX device count | 32 drivers, 32 receivers (Multidrop topology) |
Signal level | -7 V to +12 V (±200 mV sensitivity) |
Termination impedance | 120 Ω |
Receiver input resistance | At least 12 kΩ |
Despite the fact that RS-485 is an older standard, it is still used in legacy systems as it is a robust protocol. Some examples include factory floors where older systems may need to interface with a newer system over a D-sub cable (such as DB-9 or higher pin count device). Newer devices might also include an RS-485 port for interoperability with older systems when needed.
RS-485 on a Cable
The balanced signals are two lines (differential) that share a twisted pair with the same impedance on each line. In effect, this is an impedance-controlled interface, although RS-485 devices do not run at very high data rates and some have low edge rates, so short PCB traces might appear invisible to these driver/receiver pairs if connectors and RS-485 devices are placed close enough together. This is due to the input impedance seen by the interface at each section of the link, which requires parallel termination at each end of the cable.
RS-485 is still king in process automation
These signals do require have the highest bandwidth, so they can tolerate longer cable runs than you might see in a faster interface. A common cable type used for shorter distance RS-485 runs is CAT5/5e unshielded twisted pair (UTP). While CAT5 and 5e have impedance around 100 Ohms, the high signal level of this interface and the wide noise at the driver/receiver components can tolerate slight signal loss on this cable as long as the run is short enough.
RS-485 on a PCB
Technically, you could route between two RS-485 devices on the same PCB, but it is uncommon to do this and it would be better to use a standardized inter-chip interface like I2C, or possibly SPI, due to the higher data rates available. These other interfaces are ubiquitous in integrated circuits, ranging from most microcontrollers to a huge range of ASICs.
Another area where RS-485 and its cousin RS-232 are used is for a UART connection. This generally requires a level shifter to translate the differential higher-voltage RS-485 signaling format into a lower-voltage TX and RX pair used in UART connections. The UART connection could them come off the board as a pair of flying leads, as wires on a pin header, or even on another standard cable.
Just as an example, networking equipment (Ethernet switches, routers, etc.) deployed in commercial or data center environments will use one of these protocols (more commonly RS-232) coming off of a network controller or processor. The converted UART connection is then brought out through an RX/TX pair to a COM port on a computer. In these environments, it could be necessary to bring a ground connection out with the UART wires so that the receiving interface can eliminate any ground offset. Be careful with this as some of these interfaces cannot handle very large DC offsets, in which case the grounds might need to be bridged with a capacitor.Equipment in this environment may also require cable shielding to suppress EMI, as well astransient protection if a system is deployed in an industrial environment.
Wireless Technology Won't Eliminate Wired Communication
In recent years, wireless communicationhas become standard in most devices, ranging from simple home automation systems to complex embedded systems. Wireless technologies like Bluetooth, Wifi, and Zigbee currently have a huge presence in certain industries and have replaced many wired solutions.
Wireless communication solves a lot of the issues that have been troubling cable based installations. The cost of raw materials is increasing, which makes copper wire an expensive solution that end users may want to avoid. Environmental factors are another reason wireless might be preferred in certain settings. For example, outdoor installations, exposed cable runs are susceptible to crushing, extreme temperatures, moisture, dust, and any number of other factors. Lightning strikes are another culprit for breakdowns of cable runs. This results in hefty repair costs and a major inconvenience. Costly surge protectors are often installed to prolong the lifetime of devices in locations prone to lightning strikes. Alternatively, wireless repeaters have a considerably longer lifetime since they do not have data cables that can be directly exposed to lightning strikes.
Why RS-485 and Related Interfaces Will Survive
It has been almost a decade since computers stopped featuring RS-232 and RS-485 ports on motherboards, so the protocol has become invisible in the consumer space. Instead, these interfaces have been supplanted by USB for inter-device communication. This interface have many more benefits over the old RS-485 and RS-232 interfaces, primarily higher data rates and power delivery. When cables are not present, communication is done over Bluetooth. Today, there are many microcontrollers that include these interfaces, including Bluetooth SoCs/MCUs that integrate the entire RF front-end into the chip.
Although consumer devices are no longer dependent on RS-485 and its cousin protocols, it is still a popular choice in certain industries and applications. I highlighted the example from data centers above; there are other applications where it is still used in some form because the chipsets for these applications continue to include the interface, and the semiconductor vendors have not designated these components NRND or EOL. Once that finally happens, you might start to see further decline of RS-485.
You might need to buy more repeaters for your wireless network instead of long cable runs.
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