Robust and reliable bus node design often requires
the use of external transient protection devices to protect against surge transients
that may occur in industrial environments. Since these transients have a wide
frequency bandwidth (from approximately 3MHz to 300MHz), 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 100nF to 220nF
decoupling capacitors as close as possible to the VCC and
VIO pins of transceiver, UART and/or controller ICs on the
board.
- Use at least two vias for the
ground connections of the decoupling capacitors at the power pins and the
protection devices to minimize the effective via inductance.
- Use 1kΩ to 10kΩ pull-up and
pull-down 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.