IBM 3270 Hardware User's Manual page 73

Extender Card Switch Settings
In the option booklets, ignore any instructions to
set switches on the Extender Card. Instead, set
the switches on the card to match the amount of
memory in the system unit. This table shows the
correct switch settings:
Fora System Set the Extender
with a Total Card Switches to:
Memory of: 1234
256K 1011
320K
1010
384K 1001
448K 1000
512K 0111
576K 0110
640K 0101
Now, go to the Option Instruction booklet and
follow the instructions there.
2-56 Book 1 - 3270 PC Work Station User's Guide