Appendix b model 2707, interface pin assignment, Sgnd (signal ground), Rts (request to send) – Patton electronic X.21 Interfaces User Manual
Page 17: Cts (clear to send), Dsr (data set ready), Cd (carrier detect), Dtr (data terminal ready), Llb (local line loop), Tm (test mode), Rdl (remote digital loop)
