When nothing prints
Four different faults produce the same symptom: blank paper. One observation each tells them apart, and none of the four needs an instrument.
Blank paper has four causes
They share a symptom and nothing else. Sorting them takes one observation each, and the observations are free.
The button is the best instrument you have
Hold the printer's button down. What happens next splits the problem in half:
- It feeds paper and prints a page. Power is good, the head is good, the paper is in the right way round. Everything left is on the data side: baud rate, TX and RX, or a ground that is not shared.
- Nothing happens. The printer has no 9 V. Go back to the supply, and note that this is the answer even when a light is on somewhere, because the base board runs happily at 5 V while the head does not.
That page is also the printer telling you its own baud rate, which is the number your sketch has to match.
The order to check things in
- Is the charger a 9 V PD charger? Not a computer port, not a 5 V brick. This is the most common fault by a wide margin.
- Does the button feed paper? If not, stop here; it is still the supply.
- Is the roll the right way up? Thermal paper only darkens on one side.
- Does the baud rate match the sticker? 9600 receipt, 115200 label.
- Does the base board's
TXgo to your board's RX? They cross over. - Is there a common ground? Two boards on separate supplies with no ground between them cannot communicate, however correct the other four wires are.
What a wrong command set looks like
If the printer feeds, the baud rate is right and characters still come out
wrong, check which command family you are sending. ESC/POS bytes sent to the
label printer, or 1A label commands sent to the receipt printer, produce
feeding, blank paper or fragments — because the receiving machine is reading
your command bytes as something else entirely. The
two printers do not share a
language.
When it does not work
Hold the printer's button down with paper loaded. If it feeds and prints a self-test page, power and the mechanism are both fine and the problem is on the data side. If it does nothing, the problem is the supply, and no change to the sketch will help.
Power is proven, so this is data. Check that the base board's TX reaches your board's RX and not its TX, that both share a ground, and that the baud rate matches the sticker. Those three account for nearly every case.
Look at the roll before anything else — thermal paper loaded the wrong way round after a refill prints perfectly onto the uncoated side. Scratch the paper with a fingernail: the side that marks is the side that faces the head.
The supply is sagging when the head draws its peak, or the roll has run out mid-job. Ask the printer with a status query before blaming the supply. If there is paper, try a shorter and thicker USB-C cable — a long thin one drops enough voltage under load to matter.
The board behind the handshake this kit depends on, if you want to build a 9 V supply of your own.
USB-C Breakout Board →Edit this page — content/books/thermal-printer/when-nothing-prints.mdx
Questions about this product
See what other owners have asked, and read their solutions.
This page covers several products. Choose yours to see the right discussions.
Discuss this article
Ask about this page. The answer stays here, on the page it belongs to, for whoever hits the same wall next.