Shengyuan Fang and Weihong Guan have designed the PiTray mini, a companion IO board for building clusters with the Raspberry Pi Compute Module 4 (CM4). This board enables you to seamlessly upgrade your setup from the Raspberry Pi 4B to Compute Module 4 for a similar development experience. According to its creators, the PiTray mini acts as an alternative to Pi's Compute Module 4 IO Board by adding the ability to flash OS images to eMMC on a CM4.
The board, which measures 85x56 mm, features a CM4 socket along with a standard Raspberry Pi HAT connector and an optional eMMC upgrade SD card socket. However, the PiTray mini does not include audio jacks, dual micro-HDMI, or USB3.0 hub as they are rarely used in cluster building.
To get started with the onboarding of PiTray (with eMMC) on Compute Module 4, first mount the Raspberry Pi CM4 onto the PiTray but make sure that the eMMC boot switch is OFF. After installing the Raspberry Pi USB boot, connect the board to a PC via USB Type-C cable. Then, write the disk with the OS image with Raspberry Pi imager. Finally, disconnect the board and turn ON the eMMC boot switch to connect devices like HDMI, Ethernet, and USB keyboard.
More information on the PiTray mini is available on the product's wiki page. The companion IO board is currently available for pre-order on DFRobot's website for $14.50 and is expected to start shipping at the end of February.