Card Setup
RS-485 Enable Modes
RS-485 is ideal for multi-drop or network environments. RS-485 requires a tri-state driver that will allow the electrical presence of the driver to be removed from the line. The driver is in a tri-state or high impedance condition when this occurs. Only one driver may be active at a time and the other driver(s) must be tri-stated. The output modem control signal Request To Send (RTS) is typically used to control the state of the driver. Some communication software packages refer to RS-485 as RTS enable or RTS block mode transfer.
Dip switch SW1 is used to control the RS-485 mode functions for the driver circuit. The selections are:
‘RS-422 Mode’ (Always). This is the factory default mode that ensures the transmitter is always enabled. This mode is intended for Full Duplex applications such as RS-422 or RS-485 4-wire Master mode where tri-state control of the transmitter is not required.
‘RS-485 Auto Mode’. This mode uses an onboard MicroChip PIC microcontroller to sense the baud rate and control the transmitter enable signal based on the detected baud rate. The logic assumes a minimum baud rate of 1200bps and a word length of 10 bits (1 start bit, 8 data bits, and 1 stop bit). For baud rates of 38.4K or less, line turnaround should not exceed 4 bit times, 57.6K should not exceed 10 bit times, and 115.2K should not exceed 20 bit times after successfully autobauding.
‘RS-485 RTS Enable Mode’. The ‘RTS’ enable mode uses the ‘RTS’ modem control signal to enable the RS-485 interface and provides backward compatibility with existing software products. The serial driver should be used and configured for ‘RTS’ control providing a robust RS-485 solution.
‘RS-485 Data Enable Mode’. The ‘Data’ enable mode uses the transmit data stream to enable/disable the transmitter. When the data bit is High (logic 1), the transmitter will be tri-stated and relies on the line biasing resistors to place the line in the proper state. For this reason, line termination cannot be used thus reducing the data rate and maximum cable length. This mode should only be used in applications that prevent the use of the RS-485 Auto Mode or the serial driver.
