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 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 VCC and
ground connections of decoupling capacitors and
protection devices to minimize effective via
inductance.
- Use 1kΩ to 10kΩ pull-up and pull-down resistors
for enable/SLR lines to limit noise currents in
these lines during transient events.
- Insert pulse-proof resistors into the A/Y and B/Z
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.