IBM 5280 User's Setup Procedures page 58

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9. Read the following four steps (a, b, c, and d) before
connecting the data station cable.
a. Look at the data station socket with the green label. This is
where you will connect the data station cable from an auxiliary
device.
Data Station
Socket
Green Label
b. Find a cable connector with a green label.
c. Position the cable so that the green label is to your left; then
push the cable end into the socket.