Service manual

System Overview 1-73
Figure 1–32 shows the two global port modules used in the system box. One,
the B4180-Bx, is used for the QBB in the front of the system box. The other, the
B4181-Bx, is for the QBB in the back of the system box. There is no global port
module for drawer systems; since the function of the global port is built into the
drawer backplane. For a functional description of the global port, see Section
1.7.2.
The rear global port plugs into the front of the rear QBB’s backplane. The front
global port is longer and plugs into the rear of the front QBB’s backplane. The
result is that when a system box has two QBBs in it, the global ports are in the
rear of the machine close to the H-switch or distribution module to which they
are cabled.
The front and rear global port modules are identical except for their handles.
The handle for the rear module is flush with the edge of the module while the
handle for the front module is offset from the edge to compensate for the fact
that it is plugged into the back of the front QBB’s backplane. When installing
cables to the front global port, the cables pass between the handle and the edge
of the module to reach the cable connectors on the module.
Note that the console serial bus is routed through the global port module.