IBM 5280 User's Setup Manual page 84

b. Find the printer cable. Read the label to determine which socket to
connect the cable to.
c. Line up the widest slot in the cable connector with the key in the IBM
5288 socket.
Printer Cable
Socket
/
,
/~
, "
......
_"
Key
d. Insert the connector into the socket and turn the retaining ring clockwise
until the ring is tight.
Socket (20, 21,22, or 23)
8.4. If you have more than one IBM 5256,5224, or 5225 printer to
attach directly to the IBM 5288, repeat step 8.3.
User's Setup Procedures 77