Your usb-to-ttl converter must be work without connection to the printer.
Install driver (for USB)
Run HyperTerminal (COM = new COM from usb device) - 115200, 8, no, 1, no
Hyper - running
Test - type letters. See on terminal. Short RX and TX - type letters - look at terminal again.
Reaction of response should be different.
After succes test - connect to the pinter.