Question

What does it mean to use an open collector? What role does the "pull-up" resistor play? 

Say device "A" wanted to send a "1" on the wire, what would device A need to drive? What would the other devices be driving when they weren't trying to use the bus? 

Say device "A" wanted to send a "0" on the wire. What would device A need to drive? What would the other devices be driving when they weren't trying to use the bus? i. ii. iii.

Answer 1

i) An open collector is a configuration where the base of the output transistor is driven by the circuit, the emitter is grounded and the collector is open(not connected) and the external terminal is provided outwards. This is to avoid problems when the same bus is driven by outputs of multiple devices. The transistor acts as a pull-down network(if the transistor is on, the output is zero) and the resistor connected between the supply and the collector acts as a pull-up network. To understand what is the problem if open collector configuration is not used in case of buses handled by many devices, see fig.1

let p1 and n1 forms the coms output stage of the first device and p2 and n2 forms the output stage of the second device. both the outputs are connected to the bus which is shown as o/p. Now suppose if both the devices are using the bus at the same time accidentally and say device 1 is driving a LOW and device 2 is driving a HIGH. now, n1 and p2 are conducting and they form a complete short circuit from Vcc to the gnd. This results in the burning of the transistors and might lead to the burning of the bus. In case of open collector network (see fig.2) if the device1 is driving a LOW means the transistor is conducting. And if the device 2 is driving HIGH means transistor is OFF. so as there is no current through the transistor, there is no voltage drop across the resistor and so the output is HIGH. If these two devices are accidentally driving the same bus, in this case, there is the resistor in the path from Vcc to gnd to avoid high currents. so there is no burning of the devices nor the bus.

ii) If you want to send a 1, turn off the transistor. if you don't want to send anything still turn off the transistor which is equivalent to sending 1. But all transistor which are not driving makes output high but turning off their transistor. If the device has to drive 1, it also turns off the transistor so the output is 1 but if it wants to drive a 0, it turns on the transistor and the voltage is dropped between Vcc and this transistor which is turned on by all the resistors of the stages which are not driving anything.

only if you want to drive a LOW, turn on the transistor in on other cases i.e.; if you want to drive 1 or if you don't want to drive, turn off the transistor.

iii) When a device wants to drive low, it drives a low by turning on its transistor. All other devices try to drive 1 so the current flows from Vcc through resistors of all other devices and into this transistor to gnd. So the output remains gnd only even if all other transistors are driving high, voltage is dropped across their resistors.