Application Brief

RS-485 introduction

RS-485 is one of the most widely used wired interface for industrial long-haul networks. TIA/EIA-485-A standard defines the physical layer of the RS-485 interface. End applications use RS-485 interface coupled with protocol such as Profibus, Modbus or BACnet. The main benefits of this interface include:

1. Balanced differential signaling allows for rejection of common mode noise and facilitates communications over long distances in noisy industrial environments.
2. Support of –7 to 12 V wide common mode range allows for multipoint data transmission between nodes located at different ground potential difference.
3. Data rates up to 50 Mbps at short distances or communications distance up to 1000 meters at slower data rates is possible.

Applications for RS-485 include Energy Meters, Grid Protection relay, Solar inverter, Factory automation, Motor control and Heating, ventilation, air conditioning (HVAC) systems.

Isolated RS-485 in end applications

Figure 1-1 shows a typical block diagram of AC Motor drive:

A control module typically communicates to a Programmable logic controller (PLC) over RS-485 physical layer. It is common in factory environments that the ground potential difference (GPD) between the AC drive and the PLC can be much higher than the common mode voltage range suggested in the RS-485 standard. Galvanic isolation is usually introduced in the signal and supply lines of the RS-485 bus transceiver in the drive system to break to ground loop. This technique allows to transfer data between the drive and the PLC in the presence of huge GPD. In this case, basic isolation for RS-485 link is sufficient since high voltage in power stage is already isolated from control stage. Electrical fast transient (EFT) on the RS-485 link also becomes a key concern because of inductive switching.

Figure 1-2 shows typical implementation of a Solar Inverter. RS-485 serves as a communication channel between the Inverter and the control station. The configuration and performance parameters are transferred between the Control station and the inverter over the RS-485 link.

In Figure 1-2, control module that drives the inverter stage is referenced to High voltage DC-. RS-485 link is interfaced on a human accessible connector and must be protected from high voltage to account for operator safety, mandating reinforced isolation. With increasing demand for higher conversion efficiency, solar inverters are being designed with higher DC link voltages up to 1500 V. This translates to need for higher working voltage for the isolation barrier for RS-485 communication.

Other performance considerations

Other than isolation, key requirements of the systems requiring Isolated RS-485 are:

1. Differential output voltage swing: Larger differential swing on the RS-485 interface allows for greater signal-to-noise ratio (SNR) and longer reach. PROFIBUS compliant transceivers support V_OD ≥ 2.1 V and are used in several drive and factory automation systems.
2. Robustness to transient noise: Wired interfaces running on factory floors or outdoor environments are subjected to transient noise such as electrostatic discharge, lighting strike or Electrical fast transient. A robust RS-485 link is expected to survive these harsh transients and to enable the data transfer with minimum to no data loss.
3. Glitch free power up/power down for hot plug: In an RS-485 network, it is important that existing communications on the bus between two nodes must not be disturbed when a new node is swapped in or out of the network.

New Isolated RS-485

The ISO1410 is the latest Isolated RS-485/Profibus transceiver from Texas Instruments with all the above mentioned capabilities. Figure 1-3 shows the typical application schematic. This device can operate from 1.71 V to 5.5 V on the logic side, allowing to be interfaced with low voltage FPGAs and ASICs. Wide bus side supply voltage from 3 to 5.5 V eliminates the need for regulated supply voltage. This device family is fully characterized over a wide operating ambient temperature range from –40°C to +125°C and is available in 16-pin wide body SOIC package (8-mm creepage/clearance).

The SiO2 isolation barrier supports isolation withstand voltage of 5 kV_RMS for 60 s per UL1577. It is available in basic and reinforced isolation options as shown in Table 1-1.

SiO2 based isolation barrier offers superior isolation performance that is resistant to temperature and humidity. Isolation working voltage of 1500 V_pk for 40 years lifetime is highest in the industry and allows to be used in 1500 V Solar Inverter and 690 V_AC Motor drive systems.

Other differentiating features of ISO1410 are:

1. On chip 16 kV IEC ESD (contact discharge) and 4 kV IEC EFT protection improves robustness against transient events. With proper system design and utilization of isolation barrier, external protection components on bus can be eliminated and Level 4 EMC protection can be achieved.
2. Common mode transient immunity of minimum 85 kV/us is highest in the industry. This ensures reliable data communication across isolation barrier in the presence of fast-changing common mode noise.
3. 1/8 Unit load to support up to 256 nodes on the bus.
4. Receiver Failsafe provides a logic high output under Bus Short, open or idle conditions. This also eliminates 2 biasing resistors required in traditional isolated transceivers.

Conclusion

ISO1410 device offers integration along with superior system level performance. This device has highest working voltage (1500V_pk) in the industry and is capable to tolerate very high levels of IEC ESD and EFT.

These features make it a compelling solution for most industrial applications including Solar inverter and Motor drives. This device addresses key concerns often faced by end equipment designers in isolating RS-485 ports and enables communication in harsh industrial environments.