IBM 3270 Maintenance Manual page 341

Check to see if there are other IBM 3270
Personal Computers attached to your control
unit.
ARE OTHER IBM 3270 PERSONAL
COMPUTERS ATTACHED TO THE SAME
CONTROL UNIT AS YOURS?
NO
Go to "Coaxial Cable Resistance Check"
on page 3-2800-10.
YES
3-2800-6 Maintenance Information