Pictures take time

Every dot is a bit

There is no compression and no shortcut. A raster image is sent as one bit per dot, packed eight to a byte, row after row — so a full-width image is 48 bytes per row, and the height multiplies it.

On the receipt printer at 9600 baud that arithmetic matters. Serial framing is ten bits per byte, not eight, because each byte carries a start and a stop bit as well. A 384 × 200 logo is 9,600 bytes, which is 96,000 bits, which is ten seconds of wire.

The paper, meanwhile, could have printed those 200 rows in half a second. The wire is the bottleneck, by a factor of twenty.

Which is why the label printer's baud rate matters

115200 is twelve times faster than 9600, and it turns the same image from a ten-second wait into under a second. That is the practical difference the two stickers on the backs of the machines are describing, and it is invisible until you print something bigger than a line of text.

Keep images small, or do not send them

Three things make an image cheap:

• Narrow. Width is a direct multiplier. A 200-dot-wide logo costs half of a full-width one.
• Short. So is height.
• Line art. The printer has no greys, so a photograph has to be dithered to black and white before it means anything, and dithered photographs use far more ink and far more paper than the logo somebody actually wanted.

If the thing you want on the receipt is text, send it as text. The printer's own font costs one byte per character instead of a column of bits per character, and it is sharper.

Send rows with a pause between them

Both printers can be given raster data faster than they can move it into the head, and the symptom is a row shifted sideways or an image that leans. The library shipped with the label kit puts a short delay between rows — about 10 ms on an Arduino using SoftwareSerial, 3 ms on a hardware UART — and hand-written raster code needs the same.