PC/CP120 Digital Electronics Lab

How to Cascade Encoders

Background:

Encoder Chip

Sometimes you have more inputs than can be used with a single encoder chip. For instance, if you have 16 inputs but your encoder chip only takes 8 or 10. In that case, you want to cascade the encoder chips so that instead of having two sets of three bit outputs, you have a single four bit output.
You can use the 74148 IC as the encoder in this case. It has a few additional inputs and outputs compared to the 74147. Some of these extra pins are what allow these devices to be cascaded.
Below is the schematic for how to cascade two 74148s to give a single 4 bit output. Note that while the inputs are active low, the outputs are active high.

Active low inputs

In some cases, such as this, you will be using the keypad for input to devices which use active low inputs. This means that you will want a key pressed to give a low output on the corresponding line. To do this, simply switch the common connections of the keypad and resistor array mentioned above. This means you will connect the common pin of the keypad to ground and the resistor array to V_cc (+5V).

The actual resistor value isn't critical; anything between about 100 Ω and 1 k Ω should work. If the resistors get too large, then the circuit will stop working; if the resistors get too small, there will be excessive current drawn from the circuit. Ideally you want to choose a large value that works consistently.