DESIGN NOTE RS-485 Port Protection Evaluation Board 4 INTRODUCTION This evaluation board serves as an aid in evaluating circuit Bourns three previous versions of the RS-485 Evaluation protection solutions for RS-485 serial ports. It uses a Bourns Boards (EVB1, 2 and 3) used the Bourns Model TBU-CA, which TBU High-Speed Protector (HSP), a Gas Discharge Tube (GDT) is a single-channel device. Because the RS-485 interface has Surge Arrestor and a Transient Voltage Suppressor (TVS) two lines, this new evaluation board provides further PCB area that are designed to meet the required industry standards reduction by utilizing the new Bourns Model TBU-DF HSP, on RS-485 port interfaces. The recommended Bourns TBU which is a dual-channel device. This latest evaluation board is HSP solution offers enhanced performance features over the smallest RS-485 evaluation board offering a more compact competing technologies. These performance features can help solution helping designers save valuable PCB real estate. engineers improve the surge and transient protection level on RS-485 ports, while allowing them to place the entire circuit protection solution into a smaller PCB area as compared to HOW TO CONNECT THE EVALUATION BOARD alternative solutions. The Bourns RS-485 Evaluation Board 4 FOR TEST SET-UP measures 35 mm x 25 mm x 0.85 mm, and is manufactured Connect J1A and J1B to the exposed lines. using an FR-4 PCB with nickel-gold pad plating on the top and Connect J2A and J2B to the RS-485 IC device. bottom sides. J1 J2 GND GND TBU HSP TISP1 A A TVS B B TISP2 GND GND Figure 1 RS-485 Evaluation Board 4 Schematic Figure 2 RS-485 Evaluation Board 4 Top Side Layout Table 1 RS-485 Evaluation Board 4 Bill of Materials No. Part Number Qty. Description Product 1 TBU-DF085-300-WH 1 850 V 300 mA Dual Channel Bidirectional TBU HSP TBU HSP 2 2030-42T-SM-RPLF 1 Fast Acting, 3-Electrode 420 V Gas Discharge Tube GDT 3 CDSOT23-SM712 1 SOT23 12 V Dual Bidirectional TVS Diode TVS Diode The default configuration of this board uses a TBU HSP, a GDT and a TVS diode. The board allows different configurations: One Model 2030-42T-SM-RPLF could be replaced by two Model TISP4350J3BJR-S One Model 2030-42T-SM-RPLF could be replaced by two Model TISP4500H3BJR-S One Model 2030-42T-SM-RPLF could be replaced by one Model 2036-07-SM-RPLF www.bourns.com 8/28 e/K1901ADESIGN NOTE RS-485 Port Protection Evaluation Board 4 RS-485 EVALBOARD 4 CONFIGURATION Protection of RS-485 ports are typically required in three The third scenario protects against installation errors and faults. scenarios. The first scenario is for exposed and harsh For example, 12 V or 24 V lines are frequently run together DC DC environments, such as outdoor installations where induced with the signal lines. The cable runs may include other exposed lightning surges are a threat. Customers for these types lines that can induce lightning surges onto RS-485 ports. In of applications are familiar with the ITU-T K.20/21/44 addition, there is the risk of deliberate and malicious attacks on recommendations (specifying the 10/700s voltage, surge) or RS-485 ports by unauthorized users. All of these can typically be with the Telcordia GR-1089-CORE or IEC 61000-4-5 standard taken care of with a protection solution that meets the higher (specifying the CWG 8/20 s current, 1.2/50 s voltage surges). protection levels offered in scenarios one and two. The second scenario accommodates long cable runs where This evaluation board brief will highlight options for two levels multiple lines (data and AC power) are used in the same of lightning surges that use a TISP Thyristor Surge Protector for trucking or cabling. During a fault incidence, the AC power lines the lower surge level and a GDT for the higher surge level. By may come in contact with the signal lines. These applications varying the chosen voltage, this brief will also demonstrate that require 230V tests specified in ITU-T K.20/21/44, or 120 AC power cross up to 120 V or 230 V can be accommodated AC AC AC V tests specified in Telcordia GR-1089-CORE. There are by a Bourns Model TBU-DF based RS-485 protection solution. AC applications with lower voltage application/installation threats, but where the standardized 120 V or 230 V tests are still AC AC used to test the robustness of the protection solution. Table 2 Bourns TBU High-Speed Protector (HSP) and Overvoltage Protector (OVP) Combination V AC Power BO V TBU HSP Part DRM OVP Part Number OVP Surge Rating Surge Capability Cross (V) (V) Number (1000 V/s) Capability 800 A, 8/20 s TISP4350J3BJR-S 275 350 TBU-DF055-300-WH 120 V AC 350 A, 5/310 s IEC61000-4-5, Level 4, 4 kV 1.2/50 s, 42 Ohm 500 A, 8/20 s 2030-42T-SM 360 850 TBU-DF085-300-WH 230 V AC 150 A, 5/310 s ITU K.20/21/45 TISP4500H3BJR-S 200 A, 5/310 s 350 500 TBU-DF055-300-WH 230 V AC Enhanced Lighting, 10,000 A, 8/20 s 6 kV 10/700 s, 40 Ohm 2036-07-SM 75 750 TBU-DF085-300-WH 2,000 A, 5/310 s Overvoltage Protector (OVP) Selection: Breakover voltage (V ) or impulse breakdown voltage (V ) of the overvoltage protector (OVP) BO imp should be below the maximum Peak Impulse Voltage (V ) of the TBU HSP. imp V or V of the OVP should be above the maximum voltage of AC power cross (peak of V ). BO imp AC SURGE CAPABILITY TBU HSP TISP1 RS-485 TVS TISP2 Surge Generator 1.2/50 s, 4 kV Figure 3 RS-485 Port Protection Evaluation Board Surge Test Setup www.bourns.com 8/28 e/K1901A