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and three width modules. The construction is in principle the same as describedabove: the only significant difference is that these units can containseveral cards clamped together, which thus also allows a greater number ofexternal connexions. The fronts of these units can vary in appearance dependingon what they contain: thus a unit might contain power transistors andrequire cooling fins, or it might include electron tubes which must be accessiblefrom the outside. However, as in the smaller units, the front alwayscarries a nameplate on the right-hand side. For 3-module units the cases areat present not manufactured by cold flow moulding for production reasons,but are fabricated from aluminium sheet and light alloy extruded section.An example of a unit of this type is shown in fig. 5.Fig. 7 X 2528Alarm unit occupying one width modulePower Supply and Alarm EquipmentUnits with this type of construction are shown in figs. 6 and 7. Theseunits are built up on a low chassis, beneath which there is practically onlyroom for bolt-heads and nuts. A front panel, given the same enamel finishas the bay itself, is fixed to the chassis by brackets and is provided with ahandle for removal of the unit. Plug-in connexions to the bay are providedby means of connectors of type RPV 100-108 or 120. The unit wiring is ofconventional form.The width module of 93 mm (3.66") for this form of construction has beenobtained by dividing the total available bay width by 6. There are chassis etc.for 1, 2, 3 and 4 width modules. Four bay height modules are occupied bythis construction.Bay Frame and FixturesBay Frame and CablingThe bay frame consists of two C-shaped upright members formed fromsheet metal, held together by cross-members at the top and bottom: steelsheet has been chosen because of the large number of tapped holes in theupright members. The frame is enamelled and numbered in height modulesfrom the bottom upwards along the edge bent inwards from the front (seefig. 9). The frame is provided with a number of tapped holes for each heightmodule for fixing unit supports etc.Fig. 8 X 8233Shelf construction for units primarily withprinted wiringThe figure also shows how the bay cabling is run.120