The cable must carry data. Many USB‑C cables only charge.
A computer must be on the cable when the module powers up. The module looks for one only during its first 2 seconds; with the target already powering it, it is emulating and does not look any more. Switch the target off, or press the module’s RESET button with the cable connected.
Try a port on the computer itself: some USB hubs work and others do not.
Check BOOTSEL mode: hold BOOT, press and release RESET, release BOOT. If the RP2350 volume appears, the module and the cable work: install the firmware again.
Target does not boot
Look at the LEDs:
green on: the module is emulating;
green blinking alone: nothing to emulate (no image selected, or one larger than 512 KB, and no usable rescue image);
green and orange blinking together: the firmware is for another board, so install the rp2350a file.
Unplug the USB‑C cable from the computer: with a computer on it at power-on, the module opens its volume instead of emulating.
Check the soldering, the pin mapping of the carrier and its power (Hardware Integration).
Check the image: size (512 KB at most; exactly 512 KB on the Kickstart board), and that DEFAULT.TXT names it exactly.
Adjust the timing: try other READ_BUS_TICKS values.
On an Atari ST or an Amiga, start the rescue switcher and run its self-test: it names any address or data line with a bad connection.
Image issues
Images do not appear or changes have no effect: eject the ROMEMUL volume after every change; the eject is when the firmware rebuilds its list and reads DEFAULT.TXT, RESCUE.TXT and CONFIG.TXT.
The first image of the list starts instead of the one named: the name in DEFAULT.TXT does not match a file exactly, extension included.
Hidden files: names starting with a dot or an underscore are never taken for images, so the files macOS and Windows add are ignored.
A 256 KB Kickstart does not boot: the Kickstart board needs every image doubled to 512 KB (Kickstart conversion).
INFO.TXT shows an old version after an update: the firmware writes it only when there is none. Delete it and eject.