The A and B receiver inputs have an input current of 390μA (max) at 12V and -360μA (min) at -7V, respectively. According to the RS-485 standard, the relative receiver input resistance can be calculated as 360μA/800μA = 0.45 unit load (UL). Assuming a line terminated at both ends, up to 71 (= 32UL/0.45UL) MAX22506E transceivers can be used on a multi-drop network. Note that this maximum number of nodes is calculated based on DC criteria only and does not take into consideration signal integrity and transmission line effects at high data rates.
Transient Protection
Transient events that can occur in industrial environments include electrostatic discharge (ESD), surge events (e.g., lightning strikes), and electrical fast transient (EFT) or burst events. Surge pulses typically have a longer duration and higher power-withstand requirements than EFT and ESD strikes. Test requirements and limits for ESD, surge, and burst conditions are included in the IEC 61000-4-2, IEC 61000-4-4, and IEC 6100-4-5 standards.
External ESD Protection
The MAX22506E is internally protected against electrostatic discharge (ESD) events for the levels shown in the Protection section of the Electrical Characteristics table. While this internal protection increases the robustness of the device, additional external protection might be required to meet higher ESD limits or to protect against other high-voltage transients in the final application.
Electrical Fast Transient (EFT) Events
The IEC 61000-4-4 standard outlines the voltage levels and duration of an electrical fast transient (EFT) or burst event. EFT is typically a high-frequency, high-voltage burst that is coupled on to the RS-485 cable from external high-voltage switching signals. For example, switching from nearby relays or motors can generate EFT on a RS-485 data line. Relative to surge transient events, EFT bursts generate a small amount of power, but can corrupt data along the line.
To minimize the impact of EFT on signal lines, always use bypass capacitors soldered as close to the IC as possible. Additionally, common-mode chokes and TVS diodes can help to clamp high-voltage transients. Capacitive or resistor-capacitor filters can also be added on the signal lines. Shielded cables, if available, also help to reduce interference from EFT.
Surge Protection
Surge transient events can occur during lightning strikes, for example, and are characterized by the IEC 61000-4-5 standard. Surge events generate high energy, with high voltage peaks and large currents being driven to the driver outputs and receiver inputs.
Per the IEC 61000-4-5 standard, surge pulses must be referenced to protective earth (PE). Isolate PE from the field ground and connect a high-voltage capacitor and a high-voltage resistor between the field ground and PE. Figure 13 shows the current flow through the transceiver and PCB during a surge event. The MAX22506E survived 8/20μs surge testing up to ±2kV/42Ω with a 1000pF high-voltage capacitor without the need for external TVS protection.
Figure 13. Surge Protection
Layout Guidelines
The MAX22506E is designed for robust communication in harsh industrial environments. Use the following guidelines for layout to ensure optimum performance:
Place the bypass capacitor as close to the VCC pin as possible
Use supply and ground planes to reduce trace inductance.
Place external protection (resistors, capacitors, diodes) as close to the device as possible.
Design protection components directly in the path of the driver output and receiver input signals.
Additionally, pull ground planes away from the RS-485/RS-422 data lines when operating at high data rates to reduce capacitive coupling that can slow edge rates.
