This allows standard point-to-point full duplex communications, as well as a multi-drop configuration with one master (a single QVGA Controller or a desktop computer) and multiple QVGA Controller slaves. For RS232 operation: Install the jumper shunt across the two pins closest to the crystal (the default configuration). For RS485 operation: Install the jumper shunt across the two pins closest to the J3 silk screen. The two lowest order bits in the SPCR control register, named SPR1 and SPR0, determine the data exchange frequency expressed in bits per second; this frequency is also known as the baud rate. Many terminals and PCs, however, do rely on hardware handshaking to determine when the other party (in this case the QVGA Controller) is ready to accept data. The advantage of such a multi-drop RS-232 network is that the communications are full duplex, with each communicating party capable of simultaneous transmission and reception of data.

Slave devices use the master in/slave out pin, MISO, for transmitting, and the master out/slave in pin, MOSI, for receiving data. The primary and secondary serial communications ports are accessible through the QVGA Controller's 10 pin, dual row Communications Connector (H14) and through the individual Serial 1 and Serial 2 connectors. Configured as a master device, the 68HC11 transmits bytes via the "master out/slave in" pin, MOSI. Given a properly wired network and a properly configured SPCR control register, a master device may transmit a message by simply storing the byte to the SPDR data register. The QED-Forth kernel includes pre-coded drivers that configure the SPI for maximum speed data transfer using a format that is compatible with the on-board D/A and 12 bit A/D. The QVGA 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. 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 secondary channel is very useful for debugging application programs that communicate with other computers or I/O via the primary channel. Otherwise, dip switch 5 should be OFF so that it frees bit 3 of PORTA for use as general-purpose I/O. DIP switch number 7, or turn the system off and on again. You can use the QED Board’s RS485 link to create such a multi-drop serial network. To implement this multi-drop scheme, each slave keeps its RS-232 transmitter silent until it is addressed by the master and is given permission to transmit. The QScreen Controller, however, does not implement hardware handshaking. Setting the MSTR bit initializes the QScreen as a master, and clearing the MSTR bit initializes it as a slave. 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. A serial communications cable is also supplied with QScreen Starter Kits.

If you are using the QScreen as a master device, each external SPI device will require a separate select line (/SS). We have built sophisticated instruments using the QVGA Controller that operate very reliably using multiple interrupts in addition to the software UART. In addition there is no good way to add terminations resistors at the ends of a "star" network. The QVGA Controller offers a unique addition to full duplex RS232: it can place its RS232 transmitter into a high impedance silent mode under software control. The RS-232 driver (Maxim Part No. MAX242) can be tri-stated under software control. The software routines, RS232Transmit() and RS232Silent() control the dual RS232 transmitters on the board. Connecting a standard full duplex link RS232 between two computers is the same as with a standard RS232 link, with the TxD (transmitter output) of each computer connected to the RxD (receiver input) of the other computer. The QVGA Controller makes it easy to configure a multi-drop network of QVGA Controllers controlled by a single master desktop computer. Figure 11 1 shows a typical multi-drop communications network. In a multi-drop configuration, the TxD of the master is connected to the RxD of each slave, and the RxD of the master is connected to the TxD of each slave.

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