Digital Fundamentals
Input/Output Connections
Supplement
Prepared by Mike Crompton (Revised 24 September 2008 )
Inputs to logic circuits may come from mechanical switches. Outputs are often displayed
with either single LEDs (light emitting diodes) or seven-segment LED displays. The
voltage levels for a Hi and a Lo at the inputs and outputs of gates are also important.
The simple circuit shown at right illustrates an example of
Ohm’s Law that can be puzzling. If we measure the voltage
when the wire is NOT connected (i.e. an open circuit) we would
see the supply voltage, in this case 5V. This is because there is
NO VOLTAGE DROP across the resistor. The formula (Ohm’s
Law) to calculate voltage drop is V = I x R. Since the circuit is
open there is no current flow so (I) 0A x 1000Ω (R) = 0Volts
drop so we will see the total of the supply voltage.
By comparison if we measure the voltage when the wire is
connected (a short circuit) we will get 0V. The voltage across a
short circuit is always 0V because of the same law. V = I x R and
with a short circuit R is 0Ω so V is 0V. These results are not
what is normally expected, but Ohm’s and series laws confirm it.
When the switch is open no current flows therefore there is no voltage drop across the
resistor. Since the sum of the individual voltage drops equals VSUPPLY and there is no
voltage across R, all the voltage must be across the switch. When the switch is closed it
represents zero resistance and since V = I x R the voltage across the switch is 0.005 x 0 =
0V. We tend to think of an open switch being ‘Off’ and a closed switch as being ‘On’,
but in this case the open switch would put +5V (a 1) on the I/P to a gate and a closed
switch 0V (a 0).
Individual Light Emitting Diodes are available in a variety of
colours. They can only pass current through them and light
up when the correct polarity of voltage is applied to their
terminals. For the LED to light the anode must be more
positive than the cathode. The anode is the large black
triangle in the diagram at right and the cathode is the ‘minus’
sign at the bottom. Which lead is which on the actual LED is
also shown in the diagram. When lit the LED represents
almost no resistance so an excessive amount of current will
flow and burn out the LED if precautions are not taken. The
resistor in the diagram must be included but its value can
change somewhat (100Ω is a popular value). It’s purpose is
to limit the amount of current flowing through the LED.
There are two ways to connect a LED to the output of a gate (both work to a degree but
one is the preferred way).
The configuration at right is called "current sourcing"
because the current (conventional current flow not electron
current flow) for the LED comes from the power supply into
(through) the gate and then out of the gate to the LED. i.e.
The gate is the ‘source’ of power for the LED. A 74LS-- gate
can only "source" about 0.4 mA without losing output
voltage due to voltage drop within the gate
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The method shown at right of driving the LED is called
"current sinking" since the LED current (conventional) flows
from the 5V supply into the gate output and then “sinks” to
ground through the gate. A 74LS-- gate can typically "sink"
about 8 mA (20 times more). This technique (sinking) is
preferred over the previous (sourcing). A second inverter
would be required at the gate output to allow the LED to
light when the gate O/P is Hi.
I/Ps to a logic gate should not be left unconnected or
‘Floating’. In many cases a floating I/P will automatically register a Hi (1) which is
probably the exact opposite of what was intended. This can happen when a switch is
connected to the I/P. When open the switch leaves the I/P open allowing it to ‘float’. To
prevent this the gate I/P is connected directly to the +5V supply
through a ‘Pull Up’ resistor. (Usually a 1kΩ). A switch is then
connected with one terminal to ground and the other terminal to
the gate I/P. When the switch is open there is no current flow
through the resistor and, as described above, there is therefore no
voltage drop and the gate I/P is instantly ‘pulled up’ to +5V (a
1). If the switch is closed it puts the Gate I/P directly to ground
which of course is 0V (a 0). The circuit at right shows the ‘pull
up’ configuration.
Another variation of this method of connection uses a ‘Pull
Down’ resistor. The gate I/P is connected directly to ground
through the 1kΩ resistor, one switch connection goes directly to
the +5V the other is connected to the gate I/P. When the switch
is open the gate I/P is ‘pulled down’ to ground 0V (a 0) and
when closed it is connected directly to the +5V (a 1). This is
shown at right.
The Logic Trainers must comply with
the preferred method of connecting
output LEDs, also input switches to
prevent gate I/Ps from momentarily
‘floating’. The internal wiring and
components of the trainers are
configured to satisfy these requirements.
At right is a diagram showing the actual
circuitry inside the trainer.
In the ‘real life’ experiments of the
Digital Electronics Labs, an incorrect
circuit may function for a period of time after which the chip can become overheated and
‘blow’. No professional logic circuit designer would fail to follow these conventions
when creating logic circuits, so this must also apply equally to students building circuits
in the lab exercises.
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