Seiko Smart Label Printer SLP Pro, 220 etc.
Wiring diagram for serial PC cable

RJ11 by looking at the RJ11 connector' front (cable exists at RJ11' back)
        
        / //
       / //
   /------/|
  /      / |
 /------/ /
 |123456|/
 .-| |--.
   ---
   
SLP/  PC/
RJ11  DB9
 1  ?
 2  -  4  DTR
 3  -  5  GND
 4  -  2  RxD
 5  -  3  TxD
 6  ?