Sizing and Dimensions
Vertical and horizontal directions. Height is specified by the rack unit 'U', with 1 U being 1.75 inches (44.45 mm), the same unit used with 19-inch racks. Width is specified by the horizontal pitch unit 'HP', with 1 HP being 0.20 inches (5.08 mm). A 3U subrack is 5.25 inches (133.35 mm) high and accepts a 3U Eurocard which is 100 millimetres (3.937 in) high. Another popular size is the 6U high subrack which is 10.5 inches (266.70 mm) high and accepts 6U (Double) Eurocards which are 233.35 millimetres (9.187 in) high. Note that front panel sizes are slightly less than the subrack sizes: for example a 3U front panel may only be 130 millimetres (5.118 in) high whereas the 3U subrack itself is 133.35 millimetres (5.250 in) high.
The Eurocard height starts at 100 millimetres (3.937 in) and increments by 5.25 inches (133.350 mm) increments. This arrangement allows two 3U high (100 millimetres (3.937 in)) Eurocards to be supported properly next to a 6U high (233.35 millimetres (9.187 in)) Eurocard. The 33.35 millimetres (1.313 in) space is required for the card guide structure that would be between the two 3U high cards.
Eurocards come in modular depths that start at 100 millimetres (3.937 in) and then increase in 60 millimetres (2.362 in) increments. The 160 millimetres (6.299 in) depth is the most common today, however standard hardware is available to accommodate depths of 100 millimetres (3.937 in), 160 millimetres (6.299 in), 220 millimetres (8.661 in), 280 millimetres (11.024 in), 340 millimetres (13.386 in), and 400 millimetres (15.748 in).
Read more about this topic: Eurocard (printed Circuit Board)
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