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RS485Receive() clears bit PD5 to place the transceiver in receive mode, and RS485Transmit() sets bit PD5 to place the transceiver in transmit mode. When it is low, the transceiver is in receive mode. QED-Forth includes three built-in routines to facilitate control of the RS485 transceiver. 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. Likewise, the terminal’s transmit signal TxD is connected to the QScreen Controller’s receive signal RxD1. 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. 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. 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. By connecting pairs of these handshaking signals together, the terminal or PC can be made to think that the QScreen Controller is always ready to send and receive data.

The received data byte is accessed by reading SPDR data register. After configuring the SPI system to communicate on a properly connected network of devices, sending and receiving data is as simple as writing and reading a register. Finally, for master devices, the SPR1 and SPR0 bits determine the baud rate at which data is exchanged. Once the data has been exchanged, a flag bit in the SPSR status register is set to indicate that the transfer is complete. Any of these conditions may generate an interrupt if the SPIE (SPI interrupt enable) bit in the SPCR control register is set. If the programmer has enabled the local interrupt mask for the SPI, an interrupt is recognized at this point. At any given time, only the master and a single "active" slave communicate. Remember that the /SS is active low so to select a device you need to set the pin low; otherwise the pin should idle high.

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. In general, all devices on a network should use the same phase, polarity, and baud rate clock signal. Most computers conform to IBM PC AT-compatible RS232 interfaces which use 9-pin D-Type connectors, consequently the QScreen Controller brings out its serial ports to two female 9-pin D-Type connectors. Unlike the standard RS232 protocol, RS485 allows many communicating parties to share the same 3-wire communications cable. 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. If your application requires RS485, use the primary serial port (serial1) for RS485 communications, and use the secondary serial port (Serial 2) to program and debug your application code using the RS232 protocol. The primary serial channel can operate at standard speeds up to 19200 baud and can be configured for either RS232 (the default) or RS485 operation. The maximum Serial2 communications rate is 4800 baud. In summary, the code provided for implementing the second serial port is very flexible and can be used to support dual concurrent communications ports.

The remaining "inactive" slaves may actively receive, or listen to, data on the communications line, but only one slave at a time can transmit a message. The SPIF is set when a data transfer is complete, and is cleared by a read of the SPSR status register, followed by a read or write to the SPDR data register. After a data transfer is initiated by writing to the SPDR data register, the processor may poll the SPSR status register until the SPIF flag is set. 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. Slave devices use the master in/slave out pin, MISO, for transmitting, and the master out/slave in pin, MOSI, for receiving data. In some cases, however, a sophisticated network may have device groups on a network that use different clock configurations.

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