Systems like the SMBus or I²C™ use a standard resistor to pull up
the signal levels to the positive supply rail (normally 5 V). The bus
goes low because an appliance connected to it pulls the signal to
zero via its open-collector output. The well-known problem is this
output can draw a much higher current than the pull-up resistor
can compensate. This results in a steep trailing edge, but a much
more gradually rising leading edge, whose transition in addition is
not linear but exponential. This adversely affects the duty factor
of the signals and also reduces the speed of the bus.
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