All our cards use only the I2C pins from the Raspberry Pi GPIO connector to communicate with Raspberry Pi, leaving the other 25 pins available for your use.
Cards with on-board watchdog control the power to the Raspberry from the local processor. If the watchdog is activated, power is cut off if Raspberry does not access the I2C port in the preset time interval.
Cards with RS485 tranceiver use also one serial port of the Raspberry Pi. If enough resources are available, the RS485 port might be connected to the local processor as well, and jumpers are provided to decide who is controlling the transceiver.
Cards on on-board push-button have the push-button connected to pin 37 of the GPIO connector. The push-button is also connected to the local processor, if present and enough resources are available.
Cards without a processor use I2C expanders to control the I/Os.
The only exception to all the rules is the I/O Learning Card, which uses the resources of the Raspberry Pi and is not stackable.