I have only one USB port on my older IBM, Using a "IOGEAR" model GUH274, 4 port USB 2.0 hub and "Keyspan" model USA-19HS USB to serial adapter gives me no trouble at all. On a typical day I will use the blue tooth, plugged into the 4 port USB device to talk with a plc or a serial connection, than use one of the serial port adapters to talk with a HMI, and another serial port adapter to talk with a servo. This is done with all three devices online with the laptop at the same time.
I received a tip from a Omron tech about the Keyspan USB to serial port adapter. He said he used it everyday with no trouble, unlike others he tried.
I have used the Keyspan for 2 years. Omron Plc types CPM1. CPM2A, C200HE, CS1, CJ, SERIES.
Never a connection problem.
Hope this helps