Medical isolation of universal serial bus data signals

Modern PC-based patient monitoring systems are modular, where serial bus connects the measurement units and the PC. The bus is usually a derivate of RS-485 or RS-232 serial buses. The Universal Serial Bus (USB) will probably replace RS-485 and RS-232 in some new applications. The USB specification does not specify anything about isolation, which is often required in medical communication applications. In this paper, we consider different isolation alternatives and present our implementation of medical isolation of USB data signals. We also discuss the problems we observed with maintaining compliance with the USB specification.