The COM422/485A and ICOM422/485A cards were designed for asynchronous transmission/reception over long data lines and can be used in either RS-422 or RS-485 mode. The only difference between the two models is that the communication lines on the ICOM422/485A are isolated. The card’s I/O bus address is selected by DIP switch and is not limited to AT-bus COM port addresses. Utility software included with these cards helps you select an address without fear of conflict with other installed resources.
A crystal oscillator located on the cards permits precise baud rate selection from 50 to 115,200 baud. Moreover, jumper selection provides capability for baud rates up to 460,800 baud.
Type 16550 buffered UART’s are used as the communication element. These include a 16-byte transmit/receive FIFO buffer to protect against lost data in multitasking operating systems while maintaining 100% compatibility with the original IBM serial port.
The transceivers used, type 75ALS180, 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 transceivers feature thermal shutdown. Opto-isolators on the “ICOM” model provide 500-volt isolation.
Auto-RTS Transceiver Control
In RS-485 communications, the driver must be enabled when needed and then, when done transmitting, disabled to permit all cards on the network to share a two-wire or four-wire cable. The COM422/485A has two methods to control the drivers: automatic (AUTO) and request-to-send (RTS) control. Under automatic control, the driver is enabled (and the receiver disabled) when data are ready to be transmitted. When transmission is complete, the driver remains enabled for the transmission time of one additional character and then is disabled. The card automatically adjusts its timing to the baud rate of the data. The receiver is normally enabled but is disabled during transmissions and then re-enabled after transmission is completed. (Note: For operation in half-duplex under Windows, the card must be operated in the AUTO mode.)
If you wish to operate in the conventional request-to-send (RTS) mode, your application program must set a bit to enable the driver and reset that bit to disable the driver.