Serial port adapter reads 5V data but not 3V data
RS-232 has the same 1.4~1.5v threshold as 74HCTxx logic and TTL. This standard threshold is backwards compatible to all families of TTL but implemented many ways using diodes, 2 Vbe's etc.

But if you put 3.3V Tx with Vcc= >4V the transistor is ON all the time. 

So use the same Vcc as your UART and 74HCT logic. 

Good enough for short haul and better with twisted pairs Rx/gnd, Tx/gnd. 8 turns/ft