Most of the causes of garbled characters are that the conditions for serial communication between the transmitter (instrument) and the receiver (printer) do not match.
Normally, the communication conditions of the instrument can not be changed, so the printer's DIP switches 5 to 8 are adjusted to match the instrument.
If you do not know the serial communication conditions on the instrument side, try 9600 bps, which is the most popular setting.
To set 9600 bps (8 bits, no parity), set DIP switches 5 to 8 to OFF, OFF, ON, and OFF in order.
If you still get garbled, you may need to try all the combination of 5 to 9.