This baseboard for the different versions of the Raspberry Pi Compute Module 4 is just as big as the board of the Raspberry Pi 4 (85 mm × 56 mm), but offers more features such as an SSD slot and a second camera port.
The baseboard itself does not have a processor, but is only a motherboard for the Raspberry Pi Compute Module 4. The WLAN and Bluetooth capabilities as well as the memory equipment of the RasPi 4 made with it are therefore only determined by the version of the Compute Module used.
The Chinese manufacturer Waveshare offers two versions: The CM4-IO-Base-A and -B versions differ mainly in the real-time clock with battery slot that is also available on the B version.
Both boards have a slot on the underside for an M.2 SSD card in format 2230 (30 mm length) and 2242 (42 mm length). Corresponding screw threads are available on the boards for fastening, even the matching short screws with extra flat heads are included. According to Github-User Jeff Geerling the Compute Module should also be able to boot from such SSDs. It should be noted, however, that the SSDs should not have a current consumption of more than 1.5 A.
With the baseboard, the second camera port of the RasPi-4 processor is finally available. Both ports connect to foil conductor slots in the usual format so that all RasPi camera modules can be used. The connection for an LC display is also available in the conventional format.
Right next to it there is also a plug for a fan, which, however, may only have a 5 V supply voltage. Regulating the speed by means of a PWM signal is loud despite the four-pole connection on the A version of the board Wiki to the board not possible.
However, the USB 3 ports of the original are no longer available, the board only has two USB 2 sockets and a slot to which an adapter with two additional USB 2 sockets can be connected via a foil conductor. The second HDMI connection is then also available on this adapter.
Power is supplied via a USB-C socket (5V, at least 2A, correspondingly more when using an SSD), which can also be used for data transfer to the eMMC of the Compute Module, if the module can be equipped with it.
The A version is for around 20 euros, the B version for around 33 euros Manufacturer’s shop available. Compute Module 4 is not included.