The Workings of an active low input / output Priority Encoder, the 74LS148

The Workings of an active low input / output Priority Encoder, the 74LS148

This sequence of simulations, taken from the Tina© Software suite, illustrates how priority is given

to the inputs depending on the particular input that is active in a priority encoder.

Note that in order for outputs to correspond with the state of the inputs, the input enable pin has to

be asserted: In active low logic, assertion is achieved by bringing the level on the pin to logic ‘0’.

INPUT STATE A – Disabled device. EI is not asserted.

INPUT STATE B – Device enabled ( EI = ‘0’). Output EO is asserted indicating that if there may

be other processes elsewhere that should be checked for priority. GS is not asserted => none of the

inputs {0..7} have priority, because none of them are asserted.

INPUT STATE C – Device enabled (EI = ‘0’) as before. Output EO is not asserted indicating that

there may not be priorities set beyond this IC. GS asserted => an input should be checked to see if

it has priority. Port ‘7’ is asserted. {A2..A0} reads as a ‘7’ in active low output terms.

What happens if we have both SW8 and a lower level switch such as SW5 asserted? See input state

D which follows.

INPUT STATE D – Device enabled (EI = ‘0’); Output EO is not asserted, GS asserted => an input

should be checked to see if it has priority. Port ‘7’ is asserted. We have brought SW5 into a logic ‘0’

state which sets port ‘4’ to logic ‘0’ as well this time. But {A2..A0} reads as a ‘7’ in active low output

terms. Clearly port 4 does NOT have priority over port ‘7’. This also applies to the other ports

below ‘7’ in ranking which is ALL the other ports.

INPUT STATE E – Device enabled (EI = ‘0’); Output EO is asserted, GS asserted => an input

should be checked to see if it has priority. Two switches are asserted this time.

Which ports are asserted? (Cross out the ones that are NOT)

‘7’ ‘6’ ‘5’ ‘4’ ‘3’ ‘2’ ‘1’ ‘0’

Which of the asserted ports has priority now? Port ‘X’ has priority where X = ______.

Write down what A2, A1, A0 will be (asserted logic ‘0’s): {A2, A1, A0} = { }.

DETERMINATION OF STATE E - Device enabled (EI = ‘0’); Output EO is asserted, GS

asserted => an input should be checked to see if it has priority. Three switches are asserted this

time, resulting in ports ‘4’, ‘1’ and ‘0’ being asserted. Port ‘4’ has priority over the remaining two

asserted ports. Port ‘X=4’ has priority where X=(4)_BASE10 = (100)_BASE2. {A2, A1, A0} = {L, H, H}.

TRUTH TABLE FOR 74LS148 Priority Encoder --- Complete this

Port Inputs

Enable Input

Port Assigned Priority

Output Enable

Group Select

Port Selected

‘0’

‘1’

‘2’

‘3’

‘4’

‘5’

‘6’

‘7’

none

remote