Typically with serial ports, the reason for the "Access is denied" message is because another process or ComPort device in LoggerNet is fighting for access to the same serial port resource. For example, if each generic modem is set up to communicate on a separate ComPort device in the LoggerNet network map, but those ComPort devices are all using the same serial port, this may explain the error message. Alternatively, there may be some other application that is using the serial port.