PPD

from The Free On-line Dictionary of Computing (8 July 2008)
presence detect
Parallel Presence Detect
PPD
Serial Presence Detect
SPD

   <storage> A means of identifying a memory chip to the memory
   controller logic.  The original scheme, called Parallel
   Presence Detect (PPD) used a separate pin for each bit of
   information.  As the number of pins can not be very large this
   only allowed to identify the density and the speed of the
   chips.  To allow for additional information the Serial
   Presence Detect (SPD) scheme was introduced which uses a
   serial EEPROM to store the presence detect information and
   requires only two pins (one for enabling it and one for data).

   (1998-03-07)