The PCI-ICM-1S is a single port asynchronous optically isolated serial communication card that can be installed in any 3.3V or 5V PCI slot computer. Data lines are opto-isolated from the computer and from each other to assure communication even when large common mode voltages and noise are imposed. The card supports RS-422 and RS-485 balanced-mode transmission/reception. Each card has capability to add bias voltage. (RS-485 communication requires that one transmitter in the network must supply a bias voltage to ensure a known “zero” state when all transmitters are OFF). Also, receiver inputs at each end of the network should be terminated to eliminate ringing. The card supports these options by means of jumpers on the card. Type 16550 UARTs are used as the asynchronous communication elements. These include a 16-byte transmit/receive FIFO buffer to protect against lost data in multi-tasking systems while maintaining 100 percent compatibility with the original IBM serial port. A crystal oscillator is located on the card and permits precise baud rate capability up to 115,200 baud. Higher baud rates, up to 460,800 baud, are achieved by changing a jumper on the card. The driver/receivers used, type 75176, are capable of driving extremely long communication lines at high baud rates. They can drive up to ±60 mA on balanced lines and receive inputs as low as 200 mV differential signal superimposed on common mode noise of +12V to -7V. In case of communication conflict, the driver/receivers feature thermal shutdown. When the card is first installed, the operating system will detect it as new hardware and assign it an IRQ number and base address. There are no switches to set or base addresses to assign, making it easy to use. From this point on, the card behaves as a standard COM port.