Digital Buffer

The Digital Buffer

Definition

In a previous tutorial we look at the digital Not Gate or Inverter, and we saw that the NOT gates output state is the complement, opposite or inverse of its input signal. So for example, when its input signal is "HIGH" its output state will NOT be "HIGH". When its input signal is "LOW" its output state will NOT be "LOW", in other words it inverts the signal.

Another single input logical device used a lot in electronic circuits and which is the reverse or complement of the NOT gate inverter is called a Digital Buffer, Non-inverting Buffer or simply Buffer.

A Digital Buffer is another single input device that does no invert or perform any type of logical operation on its input signal as its output exactly matches that of its input. In other words, it does nothing as its output state equals its input state. The digital buffer is a "non-inverting" device and so will give us the Boolean expression of:  Q = A.

Then we can define the logical operation of a single input Digital Buffer as being:

"If A is true, then Q is true"

The Digital Buffer

Symbol	Truth Table
A Digital Buffer	A	Q
	0	0
	1	1
Boolean Expression Q = A	Read as: A gives Q

The Digital Buffer can also be made by connecting together two NOT gates as shown below. The first will "invert" the input signal A and the second will "re-invert" it back to its original level performing a double inversion of the input.

Double Inversion using NOT Gates

You may be thinking "what is the point of a Digital Buffer", if it does not alter its input signal in any way or make any logical operations like the AND or OR gates. Why not use a piece of wire instead and that's a good point. But a non-inverting Buffer has many uses in digital electronic circuits.

Digital Buffers can be used to isolate other gates or circuits from each other or buffers can be used to drive high current loads such as transistor switches because their output drive capability is much higher than their input signal requirements. In other words buffers can be used for power amplification of a digital signal as they have what is called a high "fan-out" capability.

Buffer Fan-out Example

The Fan-out parameter of a buffer (or any digial IC) is the output driving capability or output current capability of a logic gate giving greater power amplification of the input signal. It may be necessary to connect more than just one logic gate to the output of another or to switch a high current load such as an LED, then a Buffer will allow us to do just that.

Generally the output of a logic gate is usually connected to the inputs of other gates. Each input requires a certain amount of current from the gate output to change state, so that each additional gate connection adds to the load of the gate. So the fan-out is the number of parallel loads that can be driven simultaneously by one digital buffer of logic gate. Acting as a current source a buffer can have a high fan-out rating of up to 20 gates of the same logic family.

If a digital buffer has a high fan-out rating (current source) it must also have a high "fan-in" rating (current sink) as well. However, the propagation delay of the gate deteriorates rapidly as a function of fan-in so gates with a fan-in greater than 4 should be avoided.

The "Tri-state Buffer"

As well as the standard Digital Buffer seen above, there is another type of digital Buffer circuit whose output can be "electronically" disconnected from its output circuitry when required. This type of Buffer is known as a 3-State Buffer or commonly Tri-state Buffer.

A Tri-state Buffer 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" ( EN ) signal input. This control signal can be either a logic "0" or a logic "1" type signal resulting in the Tri-state Buffer being in one state allowing its output to operate normally giving either a 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 Buffer, 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 Buffer

Symbol	Truth Table
Tri-state Buffer	Enable	A	Q
	1	0	0
	1	1	1
	0	0	Hi-Z
	0	1	Hi-Z
Read as Output = Input if Enable is equal to "1"

An Active-high Tri-state Buffer is activated when a logic level "1" is applied to its "enable" control line and the data passes through from its input to its output. When the enable control line is at logic level "0", the buffer output is disabled and a high impedance condition, Hi-Z is present on the output.

Active "LOW" Tri-state Buffer

Symbol	Truth Table
Tri-state Buffer	Enable	A	Q
	0	0	0
	0	1	1
	1	0	Hi-Z
	1	1	Hi-Z
Read as Output = Input if Enable is NOT equal to "1"

An Active-low Tri-state Buffer is the opposite to the above, and is activated when a logic level "0" is applied to its "enable" control line. The data passes through from its input to its output. When the enable control line is at logic level "1", the buffer output is disabled and a high impedance condition, Hi-Z is present on the output.

Tri-state Buffer Control

The Tri-state Buffer is used in many electronic and microprocessor circuits as they allow multiple logic devices to be connected to the same wire or bus without damage or loss of data. For example, suppose we have a data line or data bus with some memory, peripherals, I/O or a CPU connected to it. Each of these devices is capable of sending or receiving data onto this single data bus without contention.

Contention occurs when multiple devices are connected together because some want to drive their output high and some low. If these devices start to send or receive data at the same time a short circuit may occur when one device outputs to the bus a logic "1" the supply voltage, while another is set at logic level "0" or ground, resulting in a short circuit condition and possibly damage to the devices as well as loss of data.

Then, the Tri-state Buffer can be used to isolate devices and circuits from the data bus and one another. If the outputs of several Tri-state Buffers are electrically connected together Decoders are used to allow only one Tri-state Buffer to be active at any one time while the other devices are in their high impedance state. An example of Tri-state Buffers connected to a single wire or bus is shown below.

Tri-state Buffer Control

It is also possible to connect Tri-state Buffer "back-to-back" to produce a Bi-directional Buffer circuit with one "active-high buffer" connected in parallel but in reverse with one "active-low buffer". Here, the "enable" control input acts more like a directional control signal causing the data to be both read "from" and transmitted "to" the same data bus wire.

Commonly available Digital Buffer and Tri-state Buffer IC's include:

TTL Logic Types   74LS07 Hex Non-inverting Buffer 74LS17 Hex Buffer/Driver 74LS244 Octal Buffer/Line Driver 74LS245 Octal Bi-directional Buffer

CMOS Logic Types   CD4050 Hex Non-inverting Buffer CD4503 Hex Tri-state Buffer HEF40244 Octal Buffer with 3-state Output

Digital Non-inverting Buffer 7407

Octal Tri-state Buffer 74244

In the next tutorial about Digital Logic Gates, we will look at the digital Logic OR Gate function as used in both TTL and CMOS logic circuits as well as its Boolean Algebra definition and truth tables.