What Is a Low-Profile PCI Card?

Issue Details

What is a low-profile PCI card, and how does it differ electrically and mechanically from a standard PCI card?

Solution

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.

Electrical Differences
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). 

Additional Information

MD1 Low-Profile Card Dimensions
MD1 defines the shortest 32-bit PCI card length, 119.91 mm (4.721 inches) and a maximum height of 64.41 mm (2.536 inches) including the edge connector. 

MD2 Low-Profile Card Dimensions
MD2 defines the maximum length of a low profile PCI card as 167.64 mm (6.600 inches) and a maximum height of 64.41 mm (2.536 inches) including the edge connector. Any low-profile PCI card longer than the MD1 length is considered an MD2 card. MD2 is the most common low-profile card form-factor. Besides standard PCI, many PCI Express cards are often described as MD2 low-profile form-factor.

Standard (Full-Height) PCI Card Dimensions
For comparison, standard (full-height) PCI cards have a specified bracket height of 120 mm (4.70 inches) and a card height of 107 mm (4.20 inches). This height includes the card edge connector. Two lengths have been defined for full-height cards, known as full-length and half-length. Full-length PCI cards have a specified length of 312 mm (12.283 inches), and half-length PCI cards are specified as having a length of up to 175.26 mm (6.9 inches). Most modern cards are half-lengths, because full-length cards cannot fit in many PC cases. The actual dimensions of cards described as half-length or full-length are most likely lower than these maximums, and will fit any standard full-height PCI slot as long as they use a properly-located full-height bracket.

Low-Profile PCI Cards Bracket and Retention Screw Dimensions
Low-profile PCI cards have a reduced bracket height of 79.2 mm (3.118 inches), and the retention screw has also been moved 1.35 mm closer to the fold in the bracket. Due to the height difference of the metal retention bracket, low-profile PCI cards do not securely insert into standard PCI slots. The smaller bracket will not fit a standard desktop, tower, or 3U rack-mount PC case, but will fit in many newer small form-factor (SFF) desktop cases or in a 2U rack-mount case. National Instruments does not sell a bracket adapter that allows low-profile PCI cards to remain securely fastened in standard PCI slots.