PCMCIA RS-422/485 Serial Communications Card
The PCM485 is a multi-protocol serial card that supports RS-422 or RS-485 communications in a single 5V PCMCIA Type II card. The PCM485 uses a crystal controlled oscillator which permits precise selection of baud rates from 50 to 115,200.
The output transceiver used, the MAXIM 485, is capable of driving extremely long communication lines at high baud rates. It can drive up to 60mA on balanced lines with a quiescent current of 300 uA. It can receive inputs as low as 200mV differential signal superimposed on common mode noise of maximum 60 V. In case of communication conflict, the transceivers feature thermal shutdown.
Model PCM485 supports Simplex, Half-Duplex, and Full-Duplex communications in a variety of 2- and 4-wire cable connections. Simplex is the simplest form of communications with transmissions occurring only in one direction. Half-Duplex allows traffic to travel in both directions, but only one at a time. And Full-Duplex has data traveling in both directions at the same time. RS-485 communication only supports half-duplex because the same pair of signal lines are used for transmitting and receiving.
AUTO AND MANUAL TRANSCEIVER CONTROL
RS-485 communications requires the transmitter driver to be enabled and disabled as needed to allow up to 32 cards to share a communications line. The PCM485 card has two methods of controlling the driver; automatic (AUTO) and request to send (RTS) control. With automatic control, the UART is normally in the receive mode. When data are ready to be transmitted, the driver is enabled. The driver remains enabled for the transmission time of one character after data transfer is complete and then the card reverts to the receive mode. The PCM485 automatically adjusts it’s timing to the baud rate of the data.
CARD AND SOCKET SERVICES
PCMCIA Card and Socket services must be loaded on the host computer system before using the PCM485 card and is typically supplied by the PCMCIA slot provider (the laptop or adaptor manufacturer) to provide the software interface to the card slot drive. In addition, our TSR memory-resident program, described below, is required to configure the card once it is recognized by the card and socket services.
The Card and Socket services handle the PCMCIA slot software interface with your operating system when a new card is inserted (or removed) or when power is applied (or upon shut-down). Once the PCMCIA slot recognizes the PCM485 card it then appears to your application software like a card on the internal ISA bus.