The PiFace is actually an I/O extender that uses the SPI bus to extend the I/O capabilities of the Raspberry Pi with 8 extra input and 8 extra output pins.
While all 8 input and 8 ouput capabilities are routed to pins for external connection, some have a double function. 2 of the 8 output pins are used to drive a couple of Relays, and 4 of the 8 input pins are fitted with buttons on the board itself. Further the input pins are directly connected to the I/O extension IC, there is no protection for over voltage in these circuits.
The 8 output pins are driven by a ULN2803A, this means that each output pin can drive up to 500mA (externally supplied) at a maximum of 50V.
In theory you could connect 4 PiFace boards to the RPi, because the address of the SPI I/O extension chip (MCP23S17SP) can be set using 2 jumpers. In reality this will be very hard to achieve because the PiFace is a board that needs to be stacked on top of the RPi and the main connector (the one on the bottom that connects to the GPIO pins on the RPi) is not fitted with a female counter part on the top to enable stacking more boards. I guess they did not do that because the Relays are too high anyway to enable stacking.
http://www.element14.com/community/docs/DOC-52896/l/piface-get-started-guide-feb-2013
http://piface.openlx.org.uk/kernel-driver-delays-and-new-interface-board
0 comments:
Post a Comment