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If the 68HC11 is initialized as a master by setting the MSTR bit, then bit 5 of the Port D data direction register (PORTD.DIRECTION) determines whether /SS is an input or an output. If the 68HC11 is initialized as a master (by setting the MSTR bit in the SPCR control register as explained below) then bit 5 of the Port D data direction register (DDRD) determines whether /SS is an input or an output. Initializing the 68HC11 as a slave (by clearing the MSTR bit in the SPCR control register as explained below) automatically configures the /SS pin as an input. The /SS pin can be configured as either an input or an output. 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. It provides a convenient means of connecting the QVGA 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.

Connecting a standard full duplex link RS232 between two computers is the same as with a standard RS232 link, what is rs485 cable with the TxD (transmitter output) of each computer connected to the RxD (receiver input) of the other computer. The dual communications channels also provide an easy way to link systems that communicate using different serial protocols. 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. For RS232 operation: Install the jumper shunt across the two pins closest to the crystal (the default configuration). Hardware is interfaced to the SPI via three PORTD pins named SCK, MOSI, and MISO brought out to pins 7, 8, and 10 on the Wildcard Port Header (see Appendix B). Any required SPI output signals must be configured as outputs by setting the appropriate bits in the Port D data direction register which is named PORTD.DIRECTION in the QED-Forth kernel. 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.

Because all of the serial I/O routines on the QVGA Controller are revectorable, it is very easy to change the serial port in use without modifying any high level code. 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. Otherwise, dip switch 5 should be OFF so that it frees bit 3 of PORTA for use as general-purpose I/O. However, Arduino and Raspberry Pi hobbyists also use it for some of their projects when multiple peripherals need to be linked to the board. The software routines, RS232Transmit() and RS232Silent() control the dual RS232 transmitters on the board. To enable the secondary serial port, turn dip switch 5 (labeled "2COM") on the QED Board ON. The primary serial port, Serial1, is supported by the 68HC11's on-chip hardware UART, and does not require interrupts to work properly.

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. If you are running Serial2 at 4800 baud, the rest of your application must be able to function properly using the remaining portion of the CPU time. Moreover, if Serial2 is running full duplex at 4800 baud, any other interrupt service routine that takes longer than 100 µs is likely to cause a problem. 1 running at the prior established baud rate (typically 9600 baud). While the default baud rate of the primary serial port is 9600 baud, you can speed your communications and download times appreciably by switching to a faster baud rate. If you do this now, remember to move the QVGA Controller’s serial connector back to Serial Port 1, and to change the terminal’s baud rate back to 9600 baud using the "Communications" item under the terminal’s "Settings" menu. Now select the "Communications" item in the "Settings" menu of the Terminal program, and click on 1200 baud (or whatever baud rate you selected in the command above). 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 (the Make icon), and after the compilation is done, enter the Terminal Program by clicking on the terminal icon and use the "Send Text File" menu item to send GETSTART.DLF to the QVGA Controller.