Low-profile PCI cards (also known as LPPCI or half-height cards) are smaller PCI cards that were developed to standardize the design of PCI cards used in small-footprint systems. Low-profile PCI cards follow the PCI Local Bus Specification (revision 2.2 and later), and also adopt the form factor requirements from an Engineering Change Notice
in late 2000 by the PCI SIG Group
Low-profile PCI cards have the same signal protocol, electrical definitions, and configuration definitions as standard PCI cards. Just like standard PCI cards, low-profile PCI cards use the 3.3 V PCI slot. All National Instruments low-profile PCI cards are electrically backwards-compatible with any otherwise-compliant PCI slot. Mechanical Differences
The low-profile PCI card standard has added new mechanical card height and depth specifications to the existing PCI Local Bus specification. There are two defined standard lengths for low-profile PCI cards, known as MD1 (to accommodate 32-bit cards) and MD2 (to accommodate either 32-bit or 64-bit cards).