AutoDirection Circuitry
Internal circuitry in the MAX13487E/MAX13488E, in
conjunction with an external pullup resistor on A and
pulldown resistor on B (see Typical Application Circuit),
act to automatically disable or enable the driver and
receiver to keep the bus in the correct state. This
AutoDirection circuitry consists of a state machine and
an additional receive comparator that determines
whether this device is trying to drive the bus, or another
node on the network is driving the bus.
The internal state machine has two inputs:
• DI
• The current state of A-B (determined by a dedicated
differential comparator)
The state machine also has two outputs:
• DRIVER_ENABLE—Internal signal that enables and
disables the driver
• RECEIVER_ENABLE—Internal signal that is the
inverse of the DRIVER_ENABLE signal, but it can be
overridden by an external pin
When DI is low, the device always drives the bus low.
When DI is high, the device drives the bus for a short
time, then disables the driver and allows the external
pullup/pulldown resistors to hold the bus in the high
state (A-B > 200mV). During each low-to-high transition
of DI, the driver stays enabled until (A-B) > VDT, and
then disables the driver, letting the pullup/pulldown
resistors hold the A and B lines in the correct state.
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