PPS

PPS

Protocol and Parameter Selection (PPS) is a negotiation mechanism defined in ISO 7816 Part 3 that allows a card reader to request specific communication parameters from a smart card after receiving its ATR. Through PPS, the reader and card agree on the transmission protocol and data rate before any application-level APDU exchange begins.

When PPS Is Used

After the card sends its ATR, the reader analyzes the interface bytes to determine the card's capabilities. If the reader wants to switch from the default protocol or change the baud rate, it sends a PPS request. The card either confirms the request by echoing it back (PPSS byte matches) or rejects it by not responding, in which case the default ATR parameters apply.

PPS is necessary in the following scenarios:

• The card supports both T=0 and T=1 and the reader prefers T=1
• The reader wants a higher baud rate than the default 9,600 bps (by selecting a faster Fi/Di ratio)
• The card advertises multiple protocol configurations and the reader selects a specific one

PPS Frame Format

A minimal PPS exchange (protocol selection only) is just three bytes: FF 00 FF (select T=0 at default speed) or FF 01 FE (select T=1 at default speed).

Relationship to ATR Negotiation

Modern smart cards may use the "specific mode" indicated in the ATR (TA2 byte present), which tells the reader that the card insists on particular parameters and PPS should not be attempted. In "negotiable mode" (TA2 absent), the reader is free to send a PPS request. Most EMV payment cards operate in specific mode, while SIM cards and government PIV cards typically support PPS negotiation.