12.1 Layout Guidelines
Robust and reliable bus node design often requires the use of external transient protection devices in order to protect against surge transients that may occur in industrial environments. Since these transients have a wide frequency bandwidth (from approximately 3 MHz to 300 MHz), high-frequency layout techniques should be applied during PCB design.
- Place the protection circuitry close to the bus connector to prevent noise transients from propagating across the board.
- Use VCC and ground planes to provide low inductance. Note that high-frequency currents tend to follow the path of least impedance and not the path of least resistance.
- Design the protection components into the direction of the signal path. Do not force the transient currents to divert from the signal path to reach the protection device.
- Apply 100-nF to 220-nF decoupling capacitors as close as possible to the VCC pins of transceiver, UART and/or controller ICs on the board.
- Use at least two vias for VCC and ground connections of decoupling capacitors and protection devices to minimize effective via inductance.
- Use 1-kΩ to 10-kΩ pullup and pulldown resistors for enable lines to limit noise currents in these lines during transient events.
- Insert pulse-proof resistors into the A and B bus lines if the TVS clamping voltage is higher than the specified maximum voltage of the transceiver bus pins. These resistors limit the residual clamping current into the transceiver and prevent it from latching up.
- While pure TVS protection is sufficient for surge transients up to 1 kV, higher transients require metal-oxide varistors (MOVs) which reduce the transients to a few hundred volts of clamping voltage, and transient blocking units (TBUs) that limit transient current to less than 1 mA.