Manual LPCI-COM-8SM
6
RS485 Balanced Mode Operation
The card supports RS485 communications and uses differential balanced drivers for long range
and noise immunity. RS485 operation involves switchable transceivers and the ability to support
multiple devices on a single "party line". The RS485 specification defines a maximum of 32
devices on a single line. The number of devices served on a single line can be expanded by use
of "repeaters".
The card also has the capability to add load resistors to terminate the communications lines.
RS485 communications requires that one transmitter 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 biasing by default and supports
termination by jumpers on the card. If your application requires the transmitter to be un-biased,
please contact the factory.
COM Port Compatibility
Eight Type 16550 compatible UARTs incorporated within a single octal UART are used as
Asynchronous
Communication Elements (ACE). These include 64-byte transmit & receive
buffers to protect against lost data in multitasking operating systems, while maintaining 100
percent compatibility with the original IBM serial port. The system assigns the address(es).
A crystal oscillator is located on the card. This oscillator permits precise selection of baud rate
up to 115,200 or, by changing a jumper, up to 921,600 with the standard crystal oscillator.
The driver/receiver used, the SP841 in non-RS232 modes, is capable of driving extremely long
communication lines at high baud rates. It 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
+12 V or -7 V. In case of communication conflict, the driver/receivers feature thermal shutdown.
The driver/receiver used in RS232 mode is the high-speed ICL3245.
Communication Mode
The card supports Half-Duplex communications with a 2-wire cable connection. Half-Duplex
allows traffic to travel in both directions, but only one way at a time. RS485 communications
commonly use the Half-Duplex mode since they share only a single pair of wires.
Baud Rate Ranges
The card has capability for two baud rate ranges and you can select which you wish to use for
all ports on the card. One range is up to 115,200 baud and the other is up to 921,600 baud.
Note:
Refer to Table 5-1: Baud Rate Divisor Values in chapter 5 of this manual.
