He have had a lot of trouble with RS232 devices in recent years.
The trouble came down to the drive software being used in modern machines.
Newer machines were simply too fast ISTR it wasn't the bit rate so much as the UART getting hit too often and too fast.
In much older machines there was a physical UART modern PC's its a software UART and that is where the trouble lay.
As the machines have got faster the code that delayed the read and write cycles became increasingly not slow enough.
In our case the code had to be re-written to give the serial device more time and voila problem gone.
Due to legacy requirements in our industry we still support serial coms even on our latest equipment its was an HA now TOPAS requirement.
We mostly use the
Lindy RS232 to USB converter.
Cheers
Mike T