GrechTech Buffered Level Shifter (General Instructions) - GrechTech/RetroRosetta GitHub Wiki

Bi-directional buffered level shifter using 16 tri-state bus buffer gates (4x SN74AHCT125N) with configurable supply voltages in a Pi Zero form factor

Full Diagram

Full Schematic

SN74AHCT125 Logic Description

A given input is labelled with a ‘Y’ suffix, the corresponding output uses an ‘A’ Suffix, and corresponding Output-Enable pin uses an ‘E’ suffix.

Powering the board

Header J6 is used for both powering the board, and configuring the power to the IC’s

Each of the four SN74AHCT125N IC’s can be configured for input or output voltages of −0.5 V to 7 V.

There are several passthroughs for Voltage Input 1 and Ground, to forward power on to other boards.

Configuring the board

Unit A is always on, using the supply voltage input from Voltage Input 1, labelled 5V.

Unit B is Off by default and On when a jumper cap is placed on the B configuration pin pair, using the supply voltage input from Voltage Input 1, labelled 5V.

Unit C is Off by default and On when a jumper cap is placed on either the left pair or right pair of the three configuration pins labelled C. When the jumper cap is on the right pair labelled 5V it uses the supply voltage from Voltage Input 1, when the jumper cap is on the left pair labelled 3V3 it uses the supply voltage from Voltage Input 2.

Unit D is Off by default and On when a jumper cap is placed on the B configuration pin pair, using the supply voltage input from Voltage Input 2, labelled 3V3.

On the output headers J1, J2, J3 and J4, for Units A, B, C and D respectively, each has one column of signal pins and one column of ground pins.

Whenever a given output/input is in use, place a jumper cap across the corresponding output-enable (‘E’) row, to short the respective pin to ground, to enable the output. Whenever a given output/input is not in use, ensure no jumper cap is on the corresponding output-enable ‘E’ pin, and place a jumper cap across the corresponding output (‘A’) row, to short the respective output pin to ground.