RS485 Port Protection Evaluation Board and Design Note

January 07, 2019 by Paul Shepard

Bourns announced the release of the RS485 Port Protection Evaluation Board 4 and accompanying Design Note. The RS-485 Port Protection Evaluation Board 4 offers an aid in evaluating circuit protection on RS-485 serial device port solutions using Bourns® TBU® high-speed protectors (HSPs), transient-voltage suppressor (TVS), and gas discharge tube surge arrestors (GDTs), products designed to meet the requisite industry standards on RS-485 port interfaces.

The RS-485 Port Protection Evaluation Board 4 is also laid out for use with alternative overvoltage protectors (OVPs) using TISP® thyristor overvoltage protectors. Bourns says that the recommended Bourns® TBU® HSP solution provides improves performance compared to competing technologies, which can help a design engineer increase the surge and transient protection level on RS-485 ports. The improved performance is said to place the circuit protection solution into a smaller PCB area. RS-485 Port Protection Evaluation Board 4 may be ordered online or through your local Bourns sales office or authorized Bourns distributor.

The Bourns® RS-485 Evaluation Board 4 measures 35mm x 25mm x 0.85mm, and the company manufactured it using an FR-4 PCB with nickel-gold pad plating on the top and bottom sides.

Bourns says its three previous models of the RS-485 Evaluation Boards (EVB1, 2 and 3) employed the Bourns® Model TBU-CA, which is a single-channel device. The RS-485 interface has two lines and the evaluation board offers further PCB area reduction through the utilization of the new Bourns® Model TBU-DF HSP, which is a dual-channel device. The company says that this latest evaluation board is the smallest RS-485 evaluation board providing a more compact solution to help designers save valuable PCB space.

RS-485 Evaluation Board 4 Configuration

Protection of RS-485 ports are usually needed in three situations. The first situation is for harsh environments, including outdoor installations in which lighting-induced surges pose a threat.  Customers for these kinds of applications know about the ITU-T K.20/21/44 recommendations that specify the 10/700 µs voltage, surge and with the Telcordia GR-1089-CORE or IEC 61000-4-5 standard that recommends the CWG 8/20 µs current, 1.2/50 µs voltage surges.

The second situation supports long cable routes in which multiple lines including data and ac power are utilized in the same trucking or cabling. During a fault occurrence, the ac power lines may come in contact with the signal lines. These applications demand the 230Vac tests specified in ITU-T K.20/21/44, or 120Vac tests specified in Telcordia GR-1089-CORE.

The third situation guards against installation mistakes and faults. For instance, 12Vdc or 24Vdc lines are often operated together with the signal lines. The cable runs may involve other exposed lines that can induce lightning surges onto RS-485 ports. Also, unauthorized users could deliberately and maliciously attack the RS-485 ports. All of these issues can usually be solved with a protection solution that meets the higher protection levels offered in situations one and two.