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There are a variety of ways the MOSI, MISO, SCK and /SS pins on your QScreen Controller can be connected. Note that the master device outputs the clock synchronization signal SCK to the slave’s SCK which is configured as an input. The resulting signal levels on the interface cable connect the local and remote in a manner specified by a standard protocol. The most widely used protocol is RS232, a full duplex protocol with a single-ended bipolar voltage swing on the serial cable. 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. When the keyword name is received by the Silence() routine running in the slave, the slave PDQ Board executes RS485Transmit() to send an acknowledgment to the master (which should now be listening to the serial bus to accept the acknowledgment). To use a QScreen as a slave in a multi-drop network, simply define a word, (named Silence(void), for example) that when executed calls RS485Receive() to wait for any pending character transmission to complete, then disable the transmitter, and then execute a routine such as Key() to listen to the communications on the serial bus.

It provides a convenient means of connecting the QScreen Controller to a variety of peripheral devices, including analog to digital and digital to analog converters, real time clocks, and other computers which use high speed communication. 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. The serial data stream at the UART is conditioned by serial driver chips that transmit and receive the data. 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. The master and slave can then exchange data. It receives bytes sent by a slave device via the "master in/slave out" pin, MISO. Serial data is shifted out least-significant-bit first. The terminal’s serial receiver chip re-inverts the signal to its positive sense. The normal way to implement this is with a two wire system with differential signal levels. The dual communications channels also provide an easy way to link systems that communicate using different serial protocols.

A modem (modulator/demodulator) provides a way of encoding digital data as a set of audio signals that can be sent over a telephone line. In addition to its physical characteristics, RS485 cables also have specific electrical properties that allow for differential signaling, enabling reliable data transmission over longer distances compared to other communication protocols. Because differential signals have inherently better signal-to-noise properties, reliable RS422 communications can be sent over much longer distances compared to RS232. Ideally, the two ends of the cable will have a termination resistor connected across the two wires. Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance. Grounds between buildings may vary by a small voltage, but with very low impedance and hence the possibility of catastrophic currents - enough to melt signal cables, PCB traces, and transceiver devices. Each RS232 driver uses inverting logic and implements a single-ended bipolar output voltage (that is, one signal that swings above and below ground). 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.

The serial output at the UART idles at the logic high (mark) level. One to two logic-high stop bits mark the end of a character. To ensure that no two devices drive the network at the same time, it is necessary that each slave device be able to disable its own RS485 data transmitter. Rather, the transmitter and receiver must be communicating using a known baud rate, or bit frequency. So long as the error between the actual baud rate and that specified is less than 1.5% (or the error between transmitter and receiver is less than 3%) there should be no communication errors. 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 maximum sustainable baud rate on the secondary serial port is 4800 baud. On the other hand, the secondary serial port (Serial2) is implemented using hardware pins PA3 (input) and PA4 (output), and is controlled by the associated interrupts IC4/OC5 and OC4, respectively.

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