Cabinet and Rack Mounting of Electronic Equipment

A rack is a floor standing structure capable of supporting chassis of electronic equipment. Early mounting of electronic equipment consisted primarily of these open air racks.

A cabinet is simply an enclosed rack. Cabinets are used in most applications today because they are more attractive, provide more protection, and because over-heating is no longer as much of a problem.

Vertical mounting members of racks or cabinets provide mounting holes for attaching front panels and chassis slides. These holes are normally the Universal RETMA hole pattern recognized by the Electronic Industries Association. Occasionally, however, the obsolete Western Electric hole pattern is used. Both hole patterns repeat every 1 3/4 inches. For this reason, the height of standard chassis front panels are made in 1 3/4 inch increments, (less 1/32 for clearance).

IMPORTANT DESIGN CONSIDERATION: Top and bottom edge of chassis front panels should be centered on 1/2 inch hole spacing. See the illustration below.

There are two standards which determined the location of mounting slots on chassis front panels, one of which is cancelled. Mil-Std-189 (cancelled) was established over 50 years ago and was based on the Western Electric hole pattern. The Electronic Industries Association  (EIA) RS-310 specification is based on the RETMA hole pattern. The table below illustrates the two standards.

Standard Front Panel Sizes and Slot Locations

Panel Height			Mil-Std-189 (Cancelled)				EIA RS-310		Figure
Number  of RETMA Increments	Effective	Actual	A	B	C	D	A	B
1	1 3/4	1.719	1.25				1.25		1
2	3 1/2	3.469	3.00				3.00
3	5 1/4	5.219	2.25				2.25		2
4	7	6.969	4.00				4.00
5	8 3/4	8.719	2.25	1.75			5.75
6	10 1/2	10.469	3.00	2.25			3.00	2.25	3
7	12 1/4	12.219	5.75	1.75			2.25	3.50
8	14	13.969	4.00	3.50			3.00	4.00
9	15 3/4	15.719	5.75	3.50			4.75	4.00
10	17 1/2	17.469	7.50	3.50			6.50	4.00
11	19 1/4	19.219	5.75	5.25			5.75	5.25
12	21	20.969	4.00	5.25	1.75		7.50	5.25
13	22 3/4	22.791	5.75	1.75	5.25				4
14	24 1/2	24.469	7.50	5.25	1.75
15	26 1/4	26.219	5.75	7.00	1.75
16	28	27.969	4.00	3.50	5.25	1.75			5
17	29 3/4	29.719	5.75	3.50	5.25	1.75
18	31 1/2	31.469	7.50	3.50	5.25	1.75

The diagram below illustrates the recommended mounting heights of the various Chassis Trak slide models. Mounting heights shown represent the distance from the bottom of the chassis front panel to the centerline of the slide. Figures are based on MIL-STD-189 or EIA RS_310. These panels are available in increments of 1 3/4 inch (less 1/32 inch clearance between panels.

Ideally, chassis width plus thickness of the slides should equal the clear opening width of the cabinet. This table describes standard chassis widths and the clear opening widths of cabinet.