Page 36 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Unit 2: Logic Gates

Figure 2.26: Example of Open Collector Circuit Notes

Open collector configuration has far more applications. The most common is a technique called
wired-AND, where the junction works as an AND gate. See the example in Figure 2.27. The
output Y will be equal to (A OR B) AND (C NAND D). The junction will work as an AND gate.
Figure 2.27: Example of Wired-AND Using Open Collector Logic Gates

Some open collector integrated circuits include 7403 (NAND gates, same pinout as 7400), 7405
(inverters, same pinout as 7404), 7409 (AND gates, same pinout as 7408) and 7433 (NOR gates,
same pinout as 7402), just to mention a few examples.

Connect the two circuits using a modulo-2 adder (EX-OR gate, i.e. IC7486).

2.2 Tri-State Gates

A Tri-state Gate can be thought of as an input controlled switch which has an output that can
be electronically turned “ON” or “OFF” by means of an external “Control” or “Enable” signal
input. This control signal can be either logic “0” or a logic “1” type signal resulting in the Tri-state
gate being in one state allowing its output to operate normally giving either logic “0” or logic “1”
output. But when activated in the other state it disables or turns “OFF” its output producing an
open circuit condition that is neither “high” or “low”, but instead gives an output state of very
high impedance, high-Z, or more commonly Hi-Z. Then this type of device has two logic state
inputs, “0” or a “1” but can produce three different output states, “0”, “1” or “Hi-Z” which is
why it is called a “3-state” device.
There are two different types of tri-state gate, one whose output is controlled by an “Active-
HIGH” control signal and the other which is controlled by an “Active-LOW” control signal, as
shown below.
Active “HIGH” Tri-state Gates

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