PPI protocol is based upon the European Standard EN 501 70 and uses the seven layer OSI model. PPI protocol implements layers 1, 2, and 7. The remaining layers 3, 4, 5, and 6, are null layers in the PPI implementation.
There are two basic types of PPI devices: masters and slaves.
Master devices initiate all messages and thus must hold the token in order to initiate messages. A master device that does not currently hold the token can optionally be designed to respond to a PPI request as a slave.
Slave devices are not token holders and therefore can never initiate a message.