SPI

(1) Short for Serial Peripheral Interface, a full-duplex synchronous serial interface for connecting low-/medium-bandwidth external devices using four wires. SPI devices communicate using a master/slave relationship over two data lines and two control lines:

• Master Out Slave In (MOSI): supplies the output data from the master to the inputs of the slaves.
• Master In Slave Out (MISO): supplies the output data from a slave to the input of the master. It is important to note that there can be no more than one slave that is transmitting data during any particular transfer.
• Serial Clock (SCLK): a control line driven by the master, regulating the flow of data bits.
• Slave Select (SS): a control line that allows slaves to be turned on and off with hardware control.

(2) Short for System Packet Interface, a point-to-point interface standard, allows network equipment designers to develop an array of next-generation multi-service switches and routers to support multi-service traffic with aggregate bandwidths up to OC-192 (10 Gb/s) and beyond, enabling them to dramatically increase system performance. It is a family of Interoperability Agreements from the Optical Internetworking Forum (OIF) which include:

• System Packet Interface Level 5 (SPI-5): OC-768 System Interface for Physical and Link Layer Devices. (OIF documentation)
• System Packet Interface Level 4 (SPI-4) Phase 2: OC-192 System Interface for Physical and Link Layer Devices.(OIF documentation)
• System Physical Interface Level 4 (SPI-4) Phase 1: A System Interface for Interconnection Between Physical and Link Layer, or Peer-to-Peer Entities Operating at an OC-192 Rate (10 Gb/s)  (OIF documentation)
• System Packet Interface Level 3: OC-48 System Interface for Physical and Link Layer Devices (OIF documentation)