d205255e_Serial_Connections___VT100_net__and_Printer_Interface_Signals_Description
[Pin] 2 [Signal] Transmitted data [Mnemonic] TXD+ [Description] 
From VT320 Sends serial characters. Held in the mark state (-) when characters are not being sent. In modem control modes, sends data only when DSR and DTR signals are on.
Digital VT320 Serial Connections – VT100.net
Description