Setting the MSTR bit initializes the 68HC11 as a master, and clearing the MSTR bit initializes it as a slave. 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. If bit 5 of DDRD is 1, then /SS is a general purpose output that operates independently of the SPI. If bit 5 of DDRD is 0, then /SS is an input. If a star configuration is unavoidable, special RS-485 repeaters are available which bidirectionally listen for data on each span and then retransmit the data onto all other spans. RS-485 is also used in building automation as the simple bus wiring and long cable length is ideal for joining remote devices. Converters between RS-485 and RS-232 are available to allow a personal computer to communicate with remote devices.

The QScreen Controller combines an embedded computer based on the 68HC11 microcontroller with a touch panel and LCD (liquid crystal display) graphic user interface (GUI) that is ideal for instrument control and automation. Configured as a master device, the 68HC11 transmits bytes via the "master out/slave in" pin, MOSI. Configured as a master device, the QScreen transmits bytes via the "master out/slave in" pin, MOSI. It is supported by virtually all personal computers, and is the default protocol for both of the QScreen Controller’s serial ports. The primary and secondary serial communications ports are accessible through the QScreen's 10 pin, dual row Serial Header (H5) which is typically not installed, the 24 pin, dual row Field Header (H3), and through the individual DB-9 Serial 1 and Serial 2 connectors. 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. 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 Serial 1 port can be configured for either RS232 or RS485 communications at up to 19200 baud. RS232’s greatest benefit is its universality; practically all personal computers can use this protocol to send and receive serial data. 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. You can use it to communicate with other devices. 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. It is a half duplex protocol, meaning that only one party at a time may transmit data. Grounds between buildings may vary by a small voltage, but with very low impedance and hence the possibility of catastrophic currents - enough to melt signal cables, PCB traces, and transceiver devices. 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 surface mount resistor pads on the QScreen that will allow you to bring out the secondary serial port to the Field Header on pins 5-6 or 7-8 as shown with the parentheses in Table 11-3. Pads are also available to bring out the RS485 signals to the DB9 Serial 1 Connector. The RS485 connections are not brought out to the Serial 1 Connector. There are several protocols that govern the format of exchanged data, with the RS232 protocol used primarily by personal computers, and the RS485 protocol used in industrial control systems. Although the RS232 protocol specifies functions for as many as 25 pins, each communications channel requires only three for simple serial interfaces: TxD1 (transmit data), RxD1 (receive data), and DGND (digital ground). If your application requires use of the secondary serial port as well as other interrupt routines, the key is to keep the interrupt service routines short and fast. 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.

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