IBM RT PC Options Installation page 102

Installing Adapters
The following pages give detailed instructions for installing adapters in the
system unit. Some of the adapters in the numbered adapter slots have
connectors to which you can connect devices outside of the system unit.
Adapters in the four system board slots (A, B, C, and D) do not connect to
external devices.
Instructions are given from the following arrangement: the system unit is
on a table. For IBM 6150, the front of the unit is toward your right hand.
You are facing the left side of the unit with all three covers removed. For
IBM 6151, you are facing the front of the system unit with the top and rear
covers removed.
Section 3. Installing Adapters in All Models 3-7