SPI-4.2 is a version of the System Packet Interface published by the Optical Internetworking Forum.
SPI-4 is an interface for packet and cell transfer between a physical layer (PHY) device and a link layer device, for aggregate bandwidths of OC-192 Asynchronous Transfer Mode and Packet over SONET/SDH (POS), as well as 10 Gigabit Ethernet applications.
The transmit and receive data paths include, respectively, (TDCLK, TDAT[15:0],TCTL) and (RDCLK, RDAT[15:0], RCTL).
The transmit and receive FIFO status channels include (TSCLK, TSTAT[1:0]) and (RSCLK, RSTAT[1:0]) respectively.
A typical application of SPI-4.2 is to connect a framer device to a network processor.