The protocol is 7 data bits, even parity, and 1 stop bit. ASCII data transfer.
Required pin out is for 25 pin at machine #1 shield; #2 transmit; #3 receive; and #7 singal ground. Same at the PC for a 25 pin "D" connector except do NOT connect shield at PC end. Use a null modem to swap 2 and 3 or wire the cable null.
For a 9 pin at the PC #5 goes to #7 of the 25 pin.
Remember transmit must always connect to receive.