I purchase and receive a PiCam module. I read several warnings about the incorrect orientation of the flex cable would damage the CM4, PiCam and camera so I first installed CM4 to PiCam Modul w/o the camera. Previously, the CM4 was installed on Rasberry Pi CM4 IO Board as it was working fine in headless mode. When CM4 was removed and install on the PiCam Modul, the red LED stay lit and the green LED flashed a few times. However, I was unable to VCN or telnet to CM4 once installed on the PiCam Modul.
The red LED is the power LED and is intended to stay lit. The green LED is connected to PIN 21(PI_NLED_ACTIVITY) and it flashing indicates a boot error. If there is an error during booting this LED will flash error patterns which can be decoded using the look up table on the Raspberry Pi website. https://www.raspberrypi.com/documentati ... lash-codes
Without more information I can only guess the cause but here are a few suggestions:
check that RPiBOOT is not shorted because that forces booting from an RPI server(e.g. PC or a Raspberry Pi). you can find the location of RPiBOOT in the manual on page 3 https://ledato.de/download/picam_manual_eng.pdf
it might be a problem with a setup serial driver like SPI or I2C that cant initiate and keeps the pi from booting, you would see that in the error code.