The RS-485 interface is a twisted-pair bus with transceiver nodes that can transmit or receive data. The receivers (R) listen continuously. The enable line puts the drivers into a high impedance state until they are called upon to transmit. The twisted-pair line can be up to 4000 feet long and is terminated at both ends.
RS-485 and RS-422 networks have been around for many years in factory automation, industrial process control with programmable logic controllers, robotics, aircraft wiring, lighting control, video surveillance, and sensor networks. It also can be found in HVAC controls, electric meter networking, and building automation.
While Ethernet has made significant inroads into the industrial networking arena, RS-485 and RS-422 have held their own simply because unlike Ethernet, they are deterministic and can handle real-time feedback and control situations. And now, Intersil’s ISL3247xE, ISL3248xE, and ISL3249xE interface transceivers are a subtle but superior way to make these legacy RS-485 and RS-422 industrial networks even more useful.
RS-485 Networks
Established and published by the ANSI Telecommunications Industry Association/Electronic Industries Alliance (TIA/EIA), RS-485 defines the physical layer (PHY) of a serial twisted-pair bus that connects multiple nodes. Each node is typically an IC transceiver that can send or receive. The bus supports direct point-to-point connections or multi-point, multi-drop arrangements. It also must be able to handle up to 32 nodes.
The two-wire twisted-pair bus uses differential signaling with logic levels in the ±2- to ±6-V range with a common-mode voltage range of –7 to +12 V (see the figure). The minimum logic voltage levels are ±200 mV.
With a common bus, only one transmitter or driver (D) can be active at any given time. During this time, other transmitters are put into a high-impedance tri-state using the enable line on the drivers. All receivers (R) listen simultaneously. Total bus length is usually restricted to 4000 feet (1200 meters).
The overall bus length and the number of connected nodes determine the upper speed limit, which ranges from about 100 kbits/s at the maximum length to 10 Mbits/s on much shorter runs. Both ends of the bus must be terminated at both in 120 Ω, the typical impedance of the twisted-pair transmission line.
The usual mode of operation is half duplex, though full duplex can be implemented by using a second twisted pair. Some systems incorporate a third ground wire for reference.
How Intersil Improves RS-485
While most wired networking standards have pursued the high-data-speed path, the RS-485 standard has remained basically static with regard to speed. Most industrial applications do not require the higher speeds of Ethernet or optical fiber. Sensor monitoring and control signaling rarely need speeds of more than a few megahertz at the highest.
That’s why RS-485 and even the slower RS-232 connections have remained so popular. More important than speed are noise mitigation, overvoltage protection (OVP), electrostatic discharge (ESD) protection, low power consumption, and other features. Intersil’s chips provide those features and more.
First, the ISL3247xE, ISL3248xE, and ISL3249xE permit up to 128 devices on the bus, up from the 32 maximum specified by the standard. Second, the series delivers the industry’s widest common-mode range (CMR) compared to similar competitive RS-485 devices.
Driver outputs and receiver inputs are fault-protected against voltages as high as ±60 V and protected against ESD events up to ±16.5 kV, enhancing reliability in harsh environments. The devices also have a quiescent supply current of only 2.3 mA, which is 30% to 50% lower than most competitive devices.
The ISL3249xE and ISL3248xE combine both ±60-V OVP and ±25-V CMR, while the ISL3247xE is rated at ±60-V OVP and ±15-V CMR. The transceivers give designers a full range of options including slew rate limited versions, high-data-rate versions, and both half-duplex and full-duplex configurations.
The family’s full fail-safe receiver inputs ensure a logic high receiver output if the inputs are shorted, floating, or on an idle bus. This eliminates the need for bus-biasing resistors. Swapping the cable data wires is a common installation error. The ISL3248xE’s polarity reversal can remotely correct it without requiring rewiring of the cable or the connector.
While the RS-485 interface usually gets no respect in a world of faster serial interfaces, these ICs will give it new capabilities and recognition.
