- Published in issue 7/1999 on page 25
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.