The GPIO ports of the CPi-S series have the same layout as the original Raspberry Pi, but there are some differences. Some ports (RS232, 485) are already connected elsewhere and are not available on the GPIO ports. There is an internal buffer chip for ESD protection.

See pp.5 of the Compute Module 4S datasheet for more information.

The 5V pins can supply a maximum current of 1A. The 3.3V pins can supply a maximum current of 700mA. If both 5V and 3.3V rail pins are used simultaneously, their total power should be kept below 5 watts.

The panel PC's RTC is connected to I2C1, so those pins assigned to I2C1 should not be reconfigured.