Arduino serial image command for memory-card playback

Trace the request from sketch to screen

A sector-based image request passes through several distinct parts: your sketch, the installed library, the serial connection, the display's command interpreter and the display's memory card. Keeping those boundaries separate makes a blank screen easier to diagnose.

Start with a documented command that does not require the card, such as a supported screen-clear operation. If the display responds as documented, you have evidence that the basic command path works. That does not establish that image playback, card access or every library function works.

If the basic command fails, investigate the connection before changing image addresses:

• Check the display's documented supply and serial signal levels. Do not assume its serial input accepts the Arduino board's output voltage.
• Match transmit and receive connections to the documented connection diagram and provide the required signal reference.
• Use the specified baud rate, framing and reset sequence.
• Confirm that the library uses the serial interface connected to the display.

A logic-level UART and an RS232 connection are not directly interchangeable. Use the interface hardware required by the equipment documentation. Disconnect power before changing wiring, and do not use a signal pin as a display power source.

Match the firmware to the library interface

The product label alone may not identify the command interface currently installed. Different firmware packages for the same display family can expose different operations. An example written for one package may compile against an unrelated library yet send commands the display does not understand.

Record the installed firmware, library version and image-export configuration. Use documentation and software from the manufacturer or its distributors, and keep those versions together with the project.

A legacy function named printByte and a function named sdWriteByte are not interchangeable merely because both contain the word “byte.” Read their declarations and implementations. One may transmit command bytes while the other performs a different operation. The relevant replacement is the documented way to request image playback, not the closest-looking function name.

Read the export as card-location metadata

A sector-based export describes where prepared image data resides on the card. It is not necessarily an instruction to open an ordinary image file by name.

For example, an export might specify an image measuring 128 by 128 pixels, a 16-bit pixel representation and a starting sector of 0x00107A. Those dimensions describe this example asset, not a requirement for every display.

Use the metadata generated for the card contents you actually programmed. If you regenerate the card layout, the location of an image may change. An old address can remain numerically valid while pointing at unrelated data.

Also distinguish the original source image from the exported playback representation. A filename ending in .jpg does not establish that the display reads JPEG data directly at the exported sector. The preparation tool may have converted it into another representation.

This is different from ordinary file-based playback. For that workflow, signage image file troubleshooting covers image-format and export checks. Those checks do not establish whether an embedded serial image request uses the correct card address.

Combine address bytes without changing their units

Suppose the matching export lists the sector address as three bytes, most significant first:

0x00, 0x10, 0x7A

The combined value is:

sector = (0x00 × 65536) + (0x10 × 256) + 0x7A
       = 4218
       = 0x00107A

The multipliers follow from an eight-bit byte: moving one byte to the left multiplies its contribution by 256. Leading zeroes make the address width visible but do not change its numeric value.

If the library accepts one sector argument, pass 4218 or 0x00107A. These are decimal and hexadecimal forms of the same integer. Do not pass the address as the text string "0x00107A" unless the documented interface explicitly requires text.

Do not multiply the value by an assumed sector size. A sector index and a byte offset use different units. The conversion is appropriate only when the destination interface explicitly asks for a byte offset and the documented storage layout defines the conversion.

Likewise, do not reverse the address experimentally. The library may accept an ordinary numeric value and handle byte order internally. Wire order belongs to the command serializer, not to the way you write the number in the sketch.

Map the export to the image function

For a library whose documented image function takes x, y, width, height, a two-bytes-per-pixel Boolean and a sector number, the example maps as follows:

The colour-mode byte in a serial packet is not necessarily the value accepted by a higher-level function. A packet might encode 16-bit pixels as 0x10, while the function asks for a Boolean. Sixteen bits equals two bytes, but you must still confirm that true selects that representation in the installed library.

If the initialized display object is named panel, and its installed library documents exactly that argument order and Boolean meaning, an illustrative call is:

const long imageSector = 0x00107AL;
const bool usesTwoBytesPerPixel = true;

panel.sdDisplayImage(
    0, 0, 128, 128,
    usesTwoBytesPerPixel,
    imageSector
);

The L suffix makes the address a long integer literal. It does not add a character or byte to the transmitted address.

Check the actual function declaration before using this example. Other interfaces may use a colour-mode enumeration, different coordinate types or a different address representation. A successful compilation establishes that the call is accepted by the compiler. It does not establish that the card contains the expected image.

Compare binary bytes with the matching export

The export can provide a useful reference for the request that should leave the Arduino. For the example command format, that reference is:

40 49 00 00 80 80 10 00 10 7A

Read these as hexadecimal byte values:

This packet is an example tied to its documented interface, not a universal serial-display protocol.

Sending the printed characters 40 is different from sending one binary byte with the value 0x40. Inspect the library's serialization code or capture the transmission using equipment compatible with the electrical interface. Keep debug text off the command connection unless the display protocol explicitly permits it.

If the packet differs from the export, resolve that difference in the library call or serializer. If it matches, move downstream to the display's documented response and card handling rather than repeatedly changing the address.

Separate card faults from drawing faults

Once basic commands work and the image request matches the export, check that the card was prepared with the associated image data. Copying the original source picture onto the card does not establish that a sector-addressed request can locate it.

Check insertion, contacts and the documented card-initialization requirements. Follow any restrictions on when the card may be inserted or removed. Read command acknowledgements and error results according to the matching firmware documentation rather than treating every returned byte as success.

What appears on screen helps narrow the next check:

• No basic command response: return to power, wiring, serial settings, initialization and firmware compatibility.
• Basic commands work, but no image appears: check image-command support, card preparation, the starting sector and the documented response.
• An image appears with wrong colours: compare the exported pixel representation with the function's colour selection.
• The wrong image appears: compare the card contents with the export that supplied the address.
• The image appears briefly: check whether later drawing commands clear or cover it.

Keep the first playback attempt isolated from other drawing operations. The useful result is not merely an accepted command: it is the expected picture, at the expected origin and dimensions, with the expected colours. Once those agree, retain the matching firmware details, library version and export metadata so later content changes do not turn a known address into a guess.

Questions