3Com 3C421600A Quick Setup Manual page 21

Connecting the STACKNET Cables 1-11
Figure 1-7 STACKNET Connections (Router Unit and two Port Expansion Units)
When adding a Port Expansion or Primary Rate Access Unit to an existing
RAS 1500 system, connect the Port Expansion Unit AC power cord before
connecting the STACKNET cable. Connecting the STACKNET cable before
the AC power cord may cause existing calls to disconnect when AC
power is applied.