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When the /SS input goes low, the slave (or QScreen in this case) transfers data in response to the SCK clock input that is initiated by the master. Many terminals and PCs, however, do rely on hardware handshaking to determine when the other party (in this case the QScreen Controller) is ready to accept data. In some cases, however, a sophisticated network may have device groups on a network that use different clock configurations. Regardless of the network, however, there are only four signals used: SCK provides a synchronized clock, MOSI and MISO signals are used for data transmission and reception, and /SS configures the QScreen as a master or slave device. 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. There are a variety of ways the MOSI, MISO, SCK and /SS pins on your QScreen Controller can be connected. 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.

Rather, the UART deduces the correct time to sample the incoming signal based on the start and stop bits in the signal itself. At the start of a transmitted character, the service routine takes about 65 µs. 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. 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. This section describes the QED-Forth routines that control the RS485 transceiver, and presents some ideas that may prove useful in designing a multi-drop data exchange protocol. The QED-Forth kernel includes pre-coded drivers that configure and control the SPI for maximum speed data transfers. Although the maximum standard baud rate of the primary serial port is 19200 baud, nonstandard baud rates of over 80 Kbaud can be attained by the 68HC11's on-chip UART and the onboard RS232 driver. We can gain insight into the operation of the RS232 protocol by examining the signal connections used for the primary serial port in Table 9 6. The transmit and receive data signals carry the messages being communicated between the QScreen Controller and the PC or terminal.

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. Setting SPE (SPI enable) to 1 turns on the SPI system. 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). If you are using the QScreen as a slave device and require the /SS signal for your external SPI hardware, configure one of the Port A pins on the Field Header as an input pin. The device that initiates a data transfer is the master, and all other devices on the network are slaves. Although data byte transfers are easily executed once the network has been wired and configured properly, a carefully executed software protocol may be required to ensure data integrity. In some circumstances a one-way data flow may suffice.

To ensure that no two devices drive the network at the same time, it is necessary that each slave device be able to disable it’s own RS-485 data transmitter. When the network master wants to talk to this particular slave, it outputs the slave’s ascii name onto the serial bus. Note that the master device outputs the clock synchronization signal SCK to the slave’s SCK which is configured as an input. You can implement the slave select lines by configuring Port A pins as outputs. These signals may alternatively be redirected to the digital inputs and outputs used by the second serial port if hardware handshaking is required. The RS232 protocol provides for four handshaking signals called ready to send (RTS), clear to send (CTS), data set ready (DSR), and data terminal ready (DTR) to coordinate the transfer of information. 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. The maximum Serial2 communications rate is 4800 baud.

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