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This allows for basic error detection, in that if noise on the transmission line causes one bit to be received incorrectly, either received as a '0' when transmitted as a '1' or vice-versa, the error would be detected due to the count of '1' bits in the byte being odd when it is expected to be even, or vice-versa depending on the parity checking settings. In either of these cases, a source of noise that caused one bit to be received incorrectly would invalidate the received byte, since the total number of '1' bits would be odd rather than even. The master and slave could even exchange ascii QED-Forth operating system commands. If more than one slave tried to drive the transmit line simultaneously, their serial drivers would fight with each other for control of the bus. The QED-Forth kernel includes pre-coded drivers that configure and control the SPI for maximum speed data transfers. Pre-coded device drivers configure the SPI for a standard data format, and it is easy to customize a data format and baud rate for your application.

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. Most modems communicate using RS232 and a set of hardware handshaking signals used to regulate data flow. Once the data has been exchanged, a flag bit in the SPSR status register is set to indicate that the transfer is complete. RS485Receive() clears bit PD5 to place the transceiver in receive mode, and RS485Transmit() sets bit PD5 to place the transceiver in transmit mode. 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). The PDQ Board controls the Serial1 and Serial2 RS485 transceivers with bits PJ0 and PJ1, respectively, of PORTJ of the processor. The thru hole transistors went obsolete, so the board has been converted to all SMD except for the 74HC595s and the screw terminals. 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 remaining inactive slaves may actively receive, or listen to, data on the communications line, but only one slave at a time can transmit a message. 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. 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. At the UART, a logic high (5 volts) is called a mark, and a logic low (0 volts) is called a space. Each RS232 driver uses inverting logic and implements a single-ended bipolar output voltage (that is, one signal that swings above and below ground). A break sequence forces the serial output to a logic low (space) at the UART. The truth tables of most popular devices, starting with the SN75176, show the output signals inverted. Also, several non-serial interrupts can stack up; if they have higher priority than the serial interrupts, they will be serviced before the Serial2 interrupt routine, and again a serial input or output bit may be lost.

If you have already downloaded the program, you are ready to go. If PT is set, all transmitted bytes with a parity bit will have an odd number of total '1' bits. However, verifying correct parity of bytes received with a parity bit is currently not supported. Configured as a master device, the QScreen transmits bytes via the "master out/slave in" pin, MOSI. For example, at 4800 baud (bits per second), each bit lasts about 200 microseconds (µs), and if communications are full duplex (e.g., if the QScreen Controller echoes each incoming character), then there is a serial interrupt every 100 µs or so. Because the software UART is interrupt based, competing interrupts that prevent timely servicing of the Serial2 interrupts can cause communications errors on the secondary serial channel. If an interrupt service routine takes longer than 200 µs, then an entire serial bit will be missed, causing a communications error. To provide a convenient means of attaching two grounds to the serial cable, there are several pins (labeled GND) on the communications connector that are connected to the controller’s ground plane.

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