What is RS-485?
RS-485 (Recommended Standard 485), also known as TIA/EIA-485-A, is a standard maintained by the Electronics Industry Alliance (EIA). RS-485 is similar to the RS-232 and RS-422 standards. The RS-485 and RS-422 standards were developed to meet the needs of electronic component designers for longer cable lengths, increased throughput and control of multiple devices.
The RS-232 standard is well known because of its use with the PC. It is used for point-to-point communications.
The RS-422 standard allows up to 10 devices to be controlled and monitored with a greatly extended range. Several interconnected machines may be connected in a master/slave configuration. Machines configured to function as slaves can communicate with the master, but not each other.
The RS-485 standard expanded the number of devices and provided a way to create true multipoint communications. The multidrop networks reduced the number of wires required to connect devices to the host, but also introduced a single point of failure - the cable.
|Transmit Range (feet)||50||4,000||4,000|
|Throughput (kb/s)||20||100 - 10,000||100 - 10,000|
As more complex equipment and processes are implemented in the industry, there arises the need to leverage existing intellectual property and products. Adding wireless RS-485 compatibility to legacy systems is a key benefit realized by OEMs wanting to extend their products' capabilities and market reach.
Wireless solutions are an effective means for extending product lines without drastically redesigning existing products and systems. When radio modems are used as the wireless link, existing products and systems may be extended with no disruption to existing services.
Adding a wireless RS-485 device to an existing system can be very cost effective while providing the following benefits:
- Ease of use and deployment in new and legacy systems
- Extended range capabilities
- Ease and flexibility of installation
Use Case Scenario
A programmable logic controller (PLC) is used to monitor and control several oil wells in a Texas oil field. The PLC is located inside a central facility to monitor temperatures and pressures of each well. Adding a new well is a problem because physical obstacles make trenching the wires impractical and costly.
A wireless RS-485 solution proves more cost effective. A wireless RS-485 interface and radio modem are used to connect the well to the PLC. The advantages for the wireless connection are realized quickly when the well needs to be moved to another location. After the well is moved to its new location, the wireless connection is reestablished quickly to the network and a significant savings in labor and parts costs are realized. As new wells are drilled, wireless RS-485 modules are used to seamlessly connect the new wells into the network.
Digi Wireless RS-485 Solutions
Digi’s wireless RS-485 RF modems require minimal configuration. The most common configurations can be set using an external DIP switch. The RF modems support AT and binary commands if additional configuration is required.
The modems are drop-in wireless RS-232/485 solutions that have undergone FCC and other agency testing and are approved for use throughout the world. Integrating these wireless data communication solutions is as simple as sending serial data from a microcontroller or comm port, while the Digi module handles all of the complexities of spread spectrum transmission and reception. A wireless RS-485 link will pass data in the same form as was received so as to appear transparent on the network with a very small addition of latency.
Digi offers the following RF modems that have built-in RS-232/RS-485/RS-422 interfacing support. All of the following have built-in RS-232/485/422 interfacing.
9XTend-PKG-R (900 MHz) RS-232/485 RF Modem
- Power Output: 1mW - 1 Watt (0 – 30 dBm)
- Indoor/urban range: up to 3000’ (900 m)
- Outdoor line-of-sight range: up to 40 miles (64 km)
- Throughput Data Rate: 9600 or 115200 bps
9XTend-NEMA (900 MHz) RS-232/485 RF Modem
- Same specifications as the 9XTend-PKG-R in a NEMA-4 Rated Enclosure
9XStream-PKG-R (900 MHz) RS-232/485 RF Modem
- Power Output: 100 mW (20 dBm)
- Indoor/urban range: up to 1500’ (450 m)
- Outdoor line-of-sight range: up to 20 miles (32 km)
- Throughput Data Rate: 9600 or 19,200 bps
24XStream-PKG-R (2.4 GHz) RS-232/485 RF Modem
- Power Output: 50 mW (17 dBm)
- Indoor/urban range: up to 600’ (180 m)
- Outdoor line-of-sight range: up to 10 miles (16 km)
- Throughput Data Rate: 9600 or 19,200 bps
9XCite-PKG-R (900 MHz) RS-232/485 RF Modem
- Power Output: 4 mW (6 dBm)
- Indoor/urban range: up to 300’ (90 m)
- Outdoor line-of-sight range: up to 1000’ (300 m)
- Throughput Data Rate: 9600 or 38,400 bps