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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 RS485 standard addresses the problem of data transmission, where a balanced (differential) transmission line is used in a multi-drop (party line) configuration (or point-to-point if only two devices are on the network). A ground connection is also necessary to ensure that the communicating devices have a common voltage reference. The /SS (active-low slave select) signal enables data transfers by slave devices when it is active low. 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. From the QScreen Controller’s point of view, these three signals (TxD, RxD, and ground) are the only connections required to perform serial communications. Table 11 5 shows the connection diagram for a standard 9-pin serial cable. The connection diagram for the above circuit is also given below. Table 9-6 shows the connection diagram for a standard 9-pin serial cable. Table 9-6 Serial Cable Connections.

The primary serial port, Serial1, is supported by the 68HC11's on-chip hardware UART (sometimes called a USART), and does not require interrupts to work properly. RS485 is another protocol supported by the primary serial port on the QScreen Controller. 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. 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. In this situation, if the /SS input is pulled low while the 68HC11 is the master, the processor detects a "mode fault" (by setting a bit in the SPI status register) meaning that there is more than one master device on the SPI bus.

Any required SPI output signals must be configured as outputs, either by calling InitSPI() or by setting the appropriate bits in the Port D data direction register DDRD. If you are using the QScreen as a master device, what is rs485 cable each external SPI device will require a separate select line (/SS). Twisted pair wire with a characteristic impedance of 120 ohms is recommended with 120 ohm termination at each end of the communications line. Why is termination important in an RS485/RS422 network? Loaded driver voltage (32 nodes on the network and termination) should typically exceed ±1.5V. While the conversion is "transparent" the software must be able to communicate with the expanded network features. While handshaking is still with us, it is usually best to disable it in software (if possible) and/or "loop-back" the pairs of signals (RTS to CTS, DTR to DSR, etc.) While RS232 was rumored to be on the "way out" with the advent of many of the new communications standards, it is still alive and well today. 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.

Hardware is interfaced to the SPI via four PORTD pins named /SS, SCK, MOSI, and MISO brought out to pins 11 through 14 on the Digital I/O connector (see Appendix A). For the QScreen, /SS is not used for SPI communication because it is used to control the direction of the RS485 transceiver; you can use any digital I/O line as a /SS signal. Typical line voltage levels from the line drivers are a minimum of ±1.5 V to a maximum of about ±6 V. Receiver input sensitivity is ±200 mV. Like RS232, the data bits are transmitted in reverse order, with the least significant bit transmitted first. This automatically activates the SCK clock which synchronously transmits the data. Configured as a master device, the QScreen transmits bytes via the "master out/slave in" pin, MOSI. This signal synchronizes the exchange of bytes between the QScreen and its peripherals. Move the serial cable from the "Serial Port 1" connector to the "Serial Port 2" connector on the QScreen.