Figure 1-31 shows the TS4500 drive bay.  The front door contains single cartridge storage slots on the inside of the door that are  referred to as  tier 0 slots . Two I/O stations are installed on the front door of the base frame.  Optionally, two extra I/O stations can be installed on the front door of any Dx5 frame. The  library’s front door has a key lo ck. The key lock is the same for every front door, and the keys  are shipped with the library. The front door is shown in Figure 1-22 on page 31. Note:  The side doors of the base frames (Lx5) also have a key lock, which is the same key  lock that is used for the front door. This switch ensures that power to the cartridge accessor is switched off whenever the front  door or side door is opened. This safety component ensures that accessor movement is  stopped while the front door is open. Figure 1-32 shows the front door safety switch. 38  IBM TS4500 R6 Tape Library Guide Figure 1-32 Front door safety switch  Figure 1-31 TS4500 drive bay Front door  Door safety switch 