The driver board

What is on it

A 74HC595 in a small surface-mount package, eight resistors, and two rows of seven holes. The chip is marked SM74HC595D — one manufacturer's version of a part that has been made by everybody since 1982, and every datasheet for it says the same things about what it does.

The fourteen pins

Six of them are the chip's own interface and eight are the display. That grouping is worth holding on to, because it is also the order you wire them in: power, then the three control wires, then the eight segments.

The eight output pins are named A to H rather than QA to QH, which is what the chip's datasheet calls them. The board is renaming them after the segments they are meant to drive, and H is the decimal point — the eighth LED in the package, which has no letter of its own on the digit.

The two pins that are not there

A 74HC595 has sixteen pins. Fourteen come out to the headers; the other two are decided on the board and cannot be changed.

OE, output enable, would let you switch all eight outputs off at once without changing what is in the register. Here it is tied on. To blank the display you send a blank byte, which is a slightly different thing — the register loses the digit rather than hiding it.

SRCLR, the shift register's reset, would clear all eight cells in one pulse. Here it is tied inactive. To clear the register you shift eight zeros through it, which costs eight clock pulses and nothing else.

Neither absence matters for a display. Both are worth knowing about the moment you read somebody else's 74HC595 code and find pins your board does not have.

Eight resistors you do not have to buy

An LED wired directly across a supply takes as much current as the supply can give it, and that is bad for both. Each segment needs a resistor in series to limit it, and on a bare digit that means eight through-hole resistors and sixteen extra joints.

They are already here, in the four pairs of small black parts around the chip on the back. That is why the segment pins go straight from the board to the digit with nothing between them.

They are unmarked, so their value cannot be read off them, and it is not published anywhere. If the exact current matters to your project — and it does once you chain several boards — put a meter in series with one segment and find out.