The UART Wildcard implements these optional RS232 modem handshaking signals on channel 1. The handshaking signals can be disabled and/or ignored by applications that do not need them. Remember that the /SS is active low so to select a device you need to set the pin low; otherwise the pin should idle high. The /SS (active-low slave select) is typically used to enable data transfers by slave devices when it is active low. The communications is asynchronous because no synchronizing clock signal is transmitted along with the data. The SCK pin’s synchronous clock signal has configurable phase, polarity and baud rate so that it can interface to a variety of synchronous serial devices. In general, all devices on a network should use the same phase, polarity, and baud rate clock signal. This connection may be used to limit the common-mode signal that can be impressed on the receiver inputs. The RS422 driver and receiver use separate differential conductor pairs on the serial cables, enabling full duplex communications. In RS485 mode, the RS422 transmit and receive pairs are shorted together with a pair of onboard jumpers as explained in the UART Wildcard Hardware: RS485 Jumpers section below.

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. 12 volts and outputs logic level (0 or 5 volt) signals to the UART circuitry. At the UART, a logic high (5 volts) is called a mark, and a logic low (0 volts) is called a space. The serial output at the UART idles at the logic high (mark) level. 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. In fact, a single driver chip on the UART Wildcard is used to implement both RS422 and RS485 communications for a given serial channel. To avoid contention on the RS485 bus, the application software must assure that only one transmitter is enabled at a time. The PT bit, with mask 0x01, determines whether even parity or odd parity is used if parity bit generation is enabled. For seven data bits with a parity bit, M would be cleared (equal to zero), and PE would be set in order to make the most-significant bit of a normal eight-bit byte be used by the serial port as a parity bit.

The words port and channel are used interchangeably to refer to a serial communications link. There are a variety of ways the MOSI, MISO, SCK and /SS pins on your QScreen Controller can be connected. A single master can broadcast commands to all the slaves, and can direct commands to an individual slave using its unique address. The master and slave could even exchange ascii QED-Forth operating system commands. This section describes the driver routines that control the RS485 transceiver, and presents some ideas that may prove useful in designing a multi-drop data exchange protocol. In the most common multi-drop RS485 protocol, one computer is designated as a master and the rest of the computers or devices on the serial bus are designated as slaves. RS232’s greatest benefit is its universality; practically all personal computers can use this protocol to send and receive serial data. The most widely used protocol is RS232, a full duplex protocol with a single-ended bipolar voltage swing on the serial cable. 5 Volt supply by a dual RS232 driver chip that has a built-in charge pump voltage multiplier. A ground connection is also necessary to ensure that the communicating devices have a common voltage reference.

Rather, the transmitter and receiver must be communicating using a known baud rate, or bit frequency. These steps greatly reduce the chance that the communicating devices might be damaged by contention on the SPI bus. A write collision occurs when a byte is written to the SPI data register, SPDR, while data is being exchanged. The serial data stream at the UART is conditioned by serial driver chips that transmit and receive the data. An optional parity bit can be specified to enable error detection by the UART. If an interrupt service routine takes longer than 200 µs, then an entire serial bit will be missed, causing a communications error. Thus, as a rough approximation, operating at 4800 baud full duplex requires about 40 to 50% of the 6811's CPU time (that is, an average of approximately 40 to 50 µs service time every 100 µs). It is also able to provide realtime Full HD(1080p or 720p) live and playback image.

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