fan-out , Buffer, Open Collector, and Tri-state


Fan-out is a term that defines the maximum number of digital inputs that the output of a single logic gate can feed. Most transistor-transistor logic ( TTL ) gates can feed up to 10 other digital gates or devices. Thus, a typical TTL gate has a fan-out of 10.

In some digital systems, it is necessary for a single TTL logic gate to drive more than 10 other gates or devices. When this is the case, a device called a buffer can be used between the TTL gate and the multiple devices it must drive. A buffer of this type has a fan-out of 25 to 30. A logical inverter (also called a NOT gate) can serve this function in most digital circuits.


  • Two inverter, or NOT, gates connected in “series” so as to invert, then re-invert, a binary bit perform the function of a buffer. Buffer gates merely serve the purpose of signal amplification: taking a “weak” signal source that isn’t capable of sourcing or sinking much current, and boosting the current capacity of the signal so as to be able to drive a load.
  • Buffer circuits are symbolized by a triangle symbol with no inverter “bubble.”
  • Buffers, like inverters, may be made in open-collector output or totem pole output forms

Open Collector OR Open Drain


Three-state logic is a logic used in electronic circuits wherein a third state, the high-impedance state

Three-state logic is used to allow multiple circuits to share the same output or bus lines which may not be capable of listening to more than one device or circuit at a time. In this way, the high-impedance state acts as a selector which blocks out circuits that are not being used. As mentioned, the whole concept of the high-impedance state is to effectively remove the circuit or device’s influence from the rest of the circuit as if it were not connected at all. Putting one device on high-impedance is normally used to prevent a short circuit with the other device directly connected in the same way to the same leads, this also prevents both devices being driven at once since this may lead to unintended output or input and cause the whole circuit to malfunction.

three outputs share same bus you can enable one of them to get its value

Totem Pole

A type of output structure used with integrated circuits in which one transistor drives the output high while another transistor connected below it pulls the output low.