penelopepug, The use of "open collector" circuits is for "ORING" other devices attached to the bus. The bus would have a passive pullup resistor to source the current required to drive the bus high. That way any device on the bus could pull it low by an open collector transistor or fet there by sending the signal to all devices that are listening. On a bipolar driven bus each of the devices will "sink or source" current at any time so you can have only 1 device in this mode, otherwise you would have bus contention meaning 1 device would be "sourcing" current and 1 device would be "sinking" current at the same time. HTH

Dave Purola,
N8NTA