Abstract: An electronic unit with a mother board carrier beneath a service carrier in a housing. The carriers are independently movable between retracted use positions and forward access positions for maintenance. Service components are hence of no hindrance when removing printed circuit boards from the mother board carrier. The mother board is horizontal with the primary surface facing upwards. This latter feature together with the orientation of other printed circuit boards and of the service components minimizes the frontal area of the unit and provides pathways for front-to-rear cooling air flow. A connector arrangement connects the two carriers together only when both carriers are in their retracted use positions. Flexible lengths of cable extending between the carriers together with attendant stowage and hindrance problems are thus avoided. A retaining bar structure is included which acts at multiple positions against daughter boards to hold them securely against vibration during transporting and also in use.