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Care must be taken when changing an RS485 node from transmit mode to receive mode, or visa versa. Because a single pair of conductors is used for both transmission and reception, RS485 is useful for multi-drop applications in which a master communicates with multiple slave serial devices, or nodes. Note that the local and the remote must share a common ground, so a minimum of 3 wires are required for half duplex RS485 communications: a pair of transceive wires and a common ground. The resulting signal levels on the interface cable connect the local and remote in a manner specified by a standard protocol. The QScreen Controller combines an embedded computer based on the 68HC11 microcontroller with a touch panel and LCD (liquid crystal display) graphic user interface (GUI) that is ideal for instrument control and automation. The two lowest order bits in the SPCR control register, named SPR1 and SPR0, determine the data exchange frequency expressed in bits per second; this frequency is also known as the baud rate.

Finally, for master devices, the SPR1 and SPR0 bits determine the baud rate at which data is exchanged. The actual baud rate produced differs from that requested by a small error owing to rounding of an internal divisor. The UART Wildcard supports any baud rate produced by the above formula. The flexibility and power of the 68HC11’s serial peripheral interface supports high speed communication between the 68HC11 and other synchronous serial devices. The QScreen Controller has two serial communications ports: a primary serial port called Serial 1 that supports both RS232 and RS485 protocols, and a secondary serial port called Serial 2 that supports RS232. It is supported by virtually all personal computers, and is the default protocol for both of the QScreen Controller’s serial ports. The RS485 protocol uses differential data signals for improved noise immunity; thus RS485 can communicate over greater distances than RS232. Newer protocols include the full duplex RS422 and the half duplex RS485 protocols, each of which drives differential 0 to 5 volt signals on the serial cable. Because differential signals have inherently better signal-to-noise properties, reliable RS422 communications can be sent over much longer distances compared to RS232.

The RS422 receiver converts the differential signal to the 0 to 5 volt logic signal required by the UART circuitry. 12 volts and outputs logic level (0 or 5 volt) signals to the UART circuitry. 5 Volt supply by a dual RS232 driver chip that has a built-in charge pump voltage multiplier. Done that establishes the RS485 receive mode is coded such that it waits until all queued (pending) characters have been transmitted before the driver chip is taken out of transmit mode. The terminal’s serial receiver chip re-inverts the signal to its positive sense. The QScreen Controller’s transmit data signal TxD1 (pin 2 on the 9-pin serial connector) is connected to the terminal’s receive data signal RxD (pin 2 on its 9-pin connector). By polling the Port A pin or by setting up an interrupt service routine, you can configure the QScreen to ignore the SCK input when /SS is high and keep MISO in a high-impedance state so that it does not interfere with the SPI bus.

The DWOM bit (port D wired-or mode) should always be set to 0. Setting DWOM to 1 takes away the processor’s ability to pull the Port D signals high unless there is a pull-up resistor on each bit of the port. No parity means that there is no parity bit. Even parity means that the bits sum to an even number, and odd parity means that the bits sum to an odd number. The PT bit, with mask 0x01, determines whether even parity or odd parity is used if parity bit generation is enabled. High (mark) parity means that the parity bit is always logic 1 at the UART, and low (space) parity means that the parity bit is always logic 0 at the UART. However, verifying correct parity of bytes received with a parity bit is currently not supported. When PE is cleared (equal to zero), the most-significant bit of each transmitted character will be a data bit. If you are using the QScreen as a master device, each external SPI device will require a separate select line (/SS).

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