IN, OUT, and the arrow between them
Both long edges of the board carry the same seven connections. One is marked IN, one OUT, and a row of chevrons between them says which way data travels. The ten bare pads are not yours.
Seven connections, printed twice
Turn the module over and every connection it has is on the two short headers at the left edge, and again on the two at the right. The left pair is under a label reading IN, the right pair under OUT, and between them a row of chevrons points from one to the other.
The two-pin header is the segment supply: GND and 12V. The five-pin header is everything else: GND, 3V3/5V, and the three signals — DATAIN (SER), LATCH (RCLK) and CLOCK (SRCLK).
The names in brackets are the shift register chip's own, which is useful the first time you read a 74HC595 datasheet and find nothing called DATAIN.
Only one pin changes meaning
Compare the two edges and they are the same list in the same order, with one exception. Where the IN edge says DATAIN (SER), the OUT edge says (QH) DATAOUT. That is the shift register's serial output, the pin that hands bits on to whatever comes next. TI writes it QH′ in the datasheet; the board prints QH.
Everything else — ground, the logic rail, the latch, the clock, the 12 V — is the same copper on both edges. It goes straight across the board. That single fact is why a chain needs one power supply and one set of wires from your microcontroller no matter how long it gets.
Common anode, and why you can ignore that
Under the brand name the board says 4" COMMON ANODE 7-SEGMENT DISPLAY. Common anode means the eight LEDs share their positive side, which is tied to the 12 V, and a segment lights when something pulls its other end down.
In a circuit you built yourself that would matter enormously, because it inverts every byte in your table. Here it does not, because the driver stage between the shift register and the segments already accounts for it: a 1 in the byte lights a segment, the same as it would on a common-cathode display. The silkscreen is telling you what the part is, not what your code has to do about it.
The ten pads that are not for you
Along the top edge: B, A, 12V, F, G. Along the bottom: DP, C, D, E, 12V. Those are the display's own pins, straight through to the LEDs, with nothing in front of them. The warning printed beside them is not decoration — there is no resistor on those pads, and an LED with no current limit lasts about as long as it takes to notice.
When it does not work
The one marked IN. Hold the board with the printing facing you and IN is at the top left. Turn it over so the digit faces you and IN is now on the right — which is where your board ends up sitting in a finished chain.
There are, and they are the same net: one beside the 12V pin and one beside the logic rail. Wire the 12 V supply's ground to the first and your board's ground to the second and the board joins them for you, which is exactly the shared ground the display needs.
The display's own segment pins, brought out as test points: B, A, 12V, F and G along one edge, DP, C, D, E and 12V along the other. They have no current limiting in front of them, which is what the boxed warning on the back is about. Leave them alone.
You are reading the back, which is a mirror of the front. The segment legend printed there is drawn as it would look from behind, so DP appears on the left of the digit rather than the right. The letters themselves are correct.
The headers are in the bag, not on the board, and pressing a wire into a bare hole is not a connection.
Solder the headers first →Edit this page — content/books/4-inch-7-segment/in-and-out.mdx
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4″ 7-Segment LED Display with 74HC595
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