IBM Netfinity 4500R User Reference Manual page 116

Input/Output ports
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IBM® Netfinity® 4500R User's Reference
connector C. Also, the Netfinity Advanced System Management Processor shares
Serial port A with the operating system.
If you install the Advanced System Management Interconnect Cable option, it
provides two additional ports. These two ports, which are used for the RS-485
function, use a dual RJ-45 connector.
You can attach a dedicated modem to the D-shell system-management connector on
the rear of your server to communicate with the integrated Netfinity Advanced
System Management Processor.
The RS-485 function uses the RJ-45 system-management connectors. This function
enables you to connect the Advanced System Management Processors of several rack-
mounted servers so that they can communicate with each other in half-duplex mode.
Note: For this function to work properly, one of the servers must have an IBM
Netfinity Advanced System Management PCI adapter installed.