why totem pole outputs can be connected together

However with open collector (or drain) gates, a gate output at logic 0 will be sinking current drawn from the external pull up resistor REXT, and any other connected open collector gate trying to output a logic 1 will have its output transistor turned off and so will not be sourcing any current. The outputs of gates without open collectors must not be connected together, because if the outputs happen to be at opposite logic states, the gate with a logic 0 output will try to sink more current than the logic 1 gate can source, and damage will most probably occur. However open collector gates can be used to make both wired AND and wired OR functions as shown in Figs. Open collector ICs are available in most of the logic types, AND, NAND etc, with the exception of OR gates.

The maximum value of collector voltage is set by the V OH parameter of the open collector gate. Open collector gates can be used with external collector V CC supplies having a voltage typically somewhere between +1.5V to +5.5V for logic gates, Buffer ICs are also available that can operate on collector V CC supplies up to +30V. Open collector and open drain gates can therefore be used for changing the levels of an output to match the higher or lower logic levels of an input on a different family of gates, when gates of mixed families are used.

3.4.1 Simplified circuit of an Open Collector NAND Gate T3 will therefore be turned off and the external pull up resistor R EXT will pull the collector voltage of T3 up to +V, which will be at the valid logic 1 level of the next gate.įig. 3.4.1, when both inputs A and B are at logic 0, the high voltage applied to T1 base will cause it to turn on, so that T1 collector will go to near 0V and T2 will turn off.Īs T2 is off there will be virtually no current through R3 so the voltage at T3 gate will be around 0V. Instead of the normal Totem Pole output stage, the single output transistor T3 has its collector brought out to an external pin, which can be connected to an external power supply, at a different voltage to the V CC supply of the IC, via an external load resistor R EXT.

The grey area illustrates a single gate within an IC. 3.4.1 shows the internal circuit of an open collector NAND gate.

Why totem pole outputs can be connected together