IBM 3270 Manual page 218

Integrated Modem Cables
3274 Models 3JC and 5JC with an Integrated Modem
Attached to a Nonswitched Line (U.S. and Canada Only)
0
Plug the four-prong plug attached to the communication cable marked
NONSWITCHED into the four-pin receptacle connected to the nonswitched line.
3274 Model 31C
Label-NONSWITCHED
OR
3274 Model 51 C
Appendix F. 3274 Communication Cables
F~9