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Since both channels can operate simultaneously and independently, debugging can be performed while the application program is communicating via its primary channel. The dual communications channels also provide an easy way to link systems that communicate using different serial protocols. In this case, cable connections may be made to Serial 1 on either the 10-pin Serial Communications Header or the Serial 1 Connector. If an interrupt service routine takes longer than 200 µs, then an entire serial bit will be missed, causing a communications error. The Silence() routine searches the incoming serial characters for a pre-determined keyword (for example, the ascii "name" of this particular slave). 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. Although the RS232 protocol specifies functions for as many as 25 pins, each communications channel requires only three for simple serial interfaces: TxD1 (transmit data), RxD1 (receive data), and DGND (digital ground). Because differential signals have inherently better signal-to-noise properties, reliable RS422 communications can be sent over much longer distances compared to RS232.

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. These factors are discussed in more detail in The UART Wildcard: Modem Handshaking Signals. These detailed signal descriptions and cable diagrams are presented to provide complete information for those who have special communications requirements and for those who wish to make their own application-specific communications cables. If you have not yet compiled the GETSTART program and you want to do the exercises here, open GETSTART.C in your TextPad editor, click on the Make Tool, and after the compilation is done, enter Mosaic Terminal by clicking on the terminal icon and use the "Send File" menu item to send GETSTART.DLF to the QScreen Controller. The RS232 driver and receiver use separate conductors on the serial cables, rs485 cable enabling full duplex communications. 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 master is in charge of designating which receiver is on at any one time. To avoid contention on the RS485 bus, the application software must assure that only one transmitter is enabled at a time.

RS485 is another protocol supported by the primary serial port on the QScreen Controller. The primary channel’s UART translates the bit-by-bit data on the serial cable into bytes of data that can be interpreted by the QED-Forth Kernel or by your application program. The default serial routines used by the onboard kernel assume that full duplex communications are available, so you cannot use the RS485 protocol to program the controller. 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. 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. To interface devices that support synchronized serial interfaces, but are not configurable like the QScreen, determine the device’s requirements for clock phase and polarity and configure the QScreen’s CPHA and CPOL accordingly. If your computer does not have an RS-232 serial port, low cost USB-to-RS-232 serial cables are available; contact Mosaic Industries for details.

The standard C serial I/O routines such as printf(), scanf(), putchar(), and getchar() give you high level access to the serial ports. Because all of the serial I/O routines on the QScreen Controller are revectorable, it is very easy to change the serial port in use without modifying any high level code. The QScreen Controller’s kernel software contains a complete set of high level driver routines for the Serial2 port, and these functions are summarized in the Control-C Glossary. Serial 2 is implemented by a software UART in the controller’s QED-Forth Kernel that uses two of the processor’s PortA I/O pins to generate a serial communications channel. It is supported by virtually all personal computers, and is the default protocol for both of the QScreen Controller’s serial ports. The next section describes the registers that configure and control the QScreen Controller’s SPI. Likewise, the terminal’s transmit signal TxD is connected to the QScreen Controller’s receive signal RxD1.