DATAOUT to DATAIN

The queue

The shift register has eight stages and a bit enters at one end. Clock in a ninth bit and something has to happen to the first one: it falls out of the far end, onto the pin printed (QH) DATAOUT. That is the chip's serial output, which TI writes QH′ in the datasheet.

Wire that pin to the next module's DATAIN and nothing is lost. The bit carries on into the next register, which is eight stages of exactly the same thing.

Watch the byte for the 7 land in the first module and then get pushed out of it by the byte for the 3. Two shiftOut calls and two modules are loaded; the first byte travelled twice as far as the second. The same figure has a control at the foot for the PWM wire, which the last section comes back to.

What crosses the joint

Each edge has six signal pins and two power pins, and the OUT edge repeats the IN edge's list. One pair is different. Every other pin is the same copper on both edges, straight across the board.

That is why a chain needs one supply and one set of wires from your microcontroller however long it gets, and why the code for a chain is barely different from the code for one module:

void updateDisplay(const uint8_t *buf) {
  digitalWrite(LAT, LOW);
  for (int i = 0; i < NUM_DIGITS; i++)
    shiftOut(SER, CLK, MSBFIRST, buf[i]);
  digitalWrite(LAT, HIGH);
}

One more module is one more turn of the loop. The latch stays outside it, pulsed once, and every digit changes at the same instant.

Plugging them together

The boards are meant to sit edge to edge. In the picture, the first module's OUT edge is against the second module's IN edge, and the same eight connections line up pin for pin across the seam. The headers from the bag, the male strips and the female sockets, are what you join them with; short jumper wires do the same job if you want the digits spaced apart.

Turn the pair over and the two digits sit at their natural spacing with no gap, which is the point of joining at the edge rather than with flying leads.

One PWM wire dims all of them

The brightness pin crosses the joint like the clock does, so one PWM wire from your board sets every digit's brightness together, and the module count does not matter. It cuts the other way too. The pad beside the PWM pin, when soldered shut, ties that module's PWM net to the logic rail, and the joint carries that to every other module.

So in a chain there are two ways to use PWM, and they do not mix:

• Leave every pad open and drive PWM from your board. One wire, one duty cycle, every digit.
• Short one pad and wire nothing to PWM. Every digit is full on, always.

A pad shorted on one module and a PWM pin wired to the chain means your pin is driving a wire that is held high. The pad ships open, so a new chain with nothing on PWM is dark: the PWM pad is the article for that.

How long can it get

Nothing in the protocol has a limit. Every extra module is one more shiftOut before the same single latch.

The limits are ordinary ones. The 12 V supply has to carry every module in the chain, since they all draw through the join. And the wires between your microcontroller and the first module carry a clock, which is the one signal here that would rather be short.