PiXtend V2 -L- Hardware Manual
The switch "SPI_EN" should always be in the "ON" position during normal operation. The
data connection between Raspberry Pi and PiXtend is only possible then.
However, the correct setting of the switch alone does not activate the data connection
automatically. In addition, the GPIO24 (Pin 18) must be configured as an output and
switched to HIGH (3.3 V).
In the PiXtend test programs, under Linux or with CODESYS, the GPIO24 is already set
automatically to enable data transfer between the controllers.
Note!
If, however, completely own software is programmed (for advanced
users), the handling of the SPI-Enable must not be forgotten!
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Figure 55: Principle circuit diagram: SPI-Enable