SLLA424C May 2022 – July 2022 ISO1176 , ISO1176T , ISO1410 , ISO1412 , ISO1430 , ISO1432 , ISO1450 , ISO1452 , ISO15 , ISO1500 , ISO3080 , ISO3082 , ISO3086 , ISO3086T , ISO3088 , ISO35 , ISO35T , ISOW1412
RS-485 has been the industry’s most used wired communications interface for more than decades. Balanced differential signaling of RS-485 allows for rejection of common mode noise and facilitates communications over long distances in noisy industrial environments. RS-485 is a common communications port in most industrial applications such as factory automation, protection relay, energy meter, motor drives and building automation.
TIA/EIA-485-A standard defines that the compliant transceivers must work with ±7 V ground potential difference (GPD). As shown in Figure 1-1, common mode voltage on receiver bus pins (Vcm) is a sum of GPD, driver output common mode voltage (Voc) and any common mode coupled noise (Vn) to the bus pins. As the communication distance between the nodes increases leading to higher GPD or as the industrial environment becomes noisier thereby coupling more common mode noise on the bus, the common mode voltage on receiver bus pins moves out of its recommended operating condition. This can lead to data corruption or damage to the transceiver.
As shown in Figure 1-2, galvanic isolation in signal and power path for the RS-485 transceiver is a solution to this problem. Since the isolation barrier impedance (>1012 ohm) is much larger than the RS-485 receiver input impedance, any voltage with respect to GND2 appears primarily across the isolation barrier. Isolation allows signal transfer to happen between receiving MCU and floating transceiver while handling large ground potential difference between GND1 and GND2.
Historically, system designers have used optocoupler based solutions to enable communication between nodes with large GPDs. A typical implementation, as shown in Figure 1-3, requires an RS-485 transceiver, 2 high speed optocouplers to transmit and receive data, a low speed optocoupler for direction control, 2 Schmitt buffers to drive the LED of the optocoupler, 1 Schmitt trigger to clean up the slow edges from low speed optocoupler, a few resistors for correct biasing and a few bypass capacitors.
The challenge with the optocoupler based solution is the high number of external components needed to achieve isolation for the RS-485 port, which increases board area and opens potential reliability issues. A SiO2 based integrated isolated RS-485 solution provides a compact and reliable alternative to the optocoupler solution for system designers in need of compact design with lower failures in time (FIT) rates.
TI offers multiple isolated RS-485 transceivers based on different isolation rating for various end-applications. ISO1410 is a noise immune transceiver with integrated IEC ESD and EFT protection in a industry standard 16-SOIC package. ISO1410 offers 5 kVRMS isolation rating per UL 1577 and 1500 Vpk of isolation working voltage per VDE. Some applications require only basic isolation to communicate between nodes with ground potential differences higher than ± 7 V that is defined by the standard. ISO1500 integrates the complete functionality of an isolated RS-485 port in a miniature SSOP-16 package. ISO1500 offers 3 kVrms isolation rating per UL 1577 and 566 Vpk of isolation working voltage per VDE.
The figures below show the layout comparison between an optocoupler based solution (Figure 1-4), a commonly available 16-SOIC based isolated RS-485 solution (Figure 1-5) and solution using the ISO1500 (Figure 1-6). ISO1500 reduces the solution board area by up to 85% compared to the optocoupler solution and by up to 50% compared to 16-SOIC based solution.
The tiny package and robust functionality of the ISO1500 provide a compelling solution across industrial applications for designs that need ground loop basic isolation for RS-485 communications port and have limited board space available.
Reliable communication over long distances in the presence of large GPD is an important consideration while designing an RS-485 port. Traditional solutions with optocouplers consume a considerable amount of PCB board space in a world where more and more applications are moving towards miniaturization.
ISO1500 is available in a tiny 4.90 mm x 3.90 mm SSOP package and requires just 3 external components in the form of bypass capacitors on the supply pins for full functionality compared to 19 components required for optocoupler based implementation. Compact solution size along with superior isolation performance and transient-noise immunity makes this device a compelling option for space constrained industrial applications.
|Device||Optimized Parameters||Performance Trade-Off|
|ISO1410||5-kVrms Isolation, IEC ESD, IEC EFT||16-SOIC Package|
|ISOW1412||5kVrms, 500kbps isolated RS-485 transceiver with integrated DC/DC converter||Integrated low radiated emissions DC/DC converter reduces board space and simplifies designs|
|ISOW1432||5kVrms, 12Mbps isolated RS-485 transceiver with integrated DC/DC converter||Integrated low radiated emissions DC/DC converter reduces board space and simplifies designs|