Motorola 68230 - Vanguard 300 DSU/CSU Operator's Manual page 19

Cable Clearance/Air Circulation
To avoid overheating the unit's circuitry, you should never place anything on top
of the unit, within 1 inch (2.5 cm) of the ventilation slots on the front panel, or
within 12 inches (30.5 cm) of the back of the unit. Allow at least 12 inches (30.5
cm) in back of the unit for interfacing cable clearance and air circulation, as
shown in Figure 2-1.
Figure 2-1. Proper cable and air clearance