Some LM-390A units use a CH340 or PL2303 serial-to-USB chip internally.
The Letatwin LM-390A is manufactured by MAX Co., Ltd. (Japan). The software required to run the machine from a PC is typically called the Letatwin PC Editor or LM-390A Utility Software. letatwin lm-390a pc driver download
Since the official MAX Co. support pages have archived this model, users typically have two options: Some LM-390A units use a CH340 or PL2303
Assuming you have obtained a legitimate driver file (e.g., LM390A_Driver.exe or an INF-based driver), follow this procedure: Test Printing: Open Notepad or a label design software (e