For RS485 operation: Install the jumper shunt across the two pins closest to the J3 silk screen. 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. RS485 is another protocol supported by the primary serial port on the QScreen Controller. If you do this now, remember to move the QVGA Controller’s serial connector back to Serial Port 1, and to change the terminal’s baud rate back to 9600 baud using the "Communications" item under the terminal’s "Settings" menu. 19200 baud, select the "Comm" item under the "Settings" menu and click on 19200 in the "Baud Rate" selection box. 1 running at the prior established baud rate (typically 9600 baud). Thus, as a rough approximation, operating at 4800 baud full duplex requires about 40 to 50% of the 6811's CPU time (that is, an average of approximately 40 to 50 µs service time every 100 µs).

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. This allows the processor that is master to control the input /SS pins of other CPU’s, for example. The 68HC11 allows the details of the synchronous communications protocol to be customized for compatibility with a variety of peripherals. There are a variety of ways the MOSI, MISO, SCK and /SS pins on your QVGA Controller can be connected. A mode fault occurs when the SPI senses that a multimaster conflict (MC68HC11F1 Technical Data Manual, p.10-5) exists on the network as explained above in connection with the /SS input. 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. Clearing the MSTR bit in the SPCR control register automatically configures the /SS (slave select) pin as an input. 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. The QVGA Controller, however, does not implement hardware handshaking. RS-485 is used as the physical layer underlying many standard and proprietary automation protocols used to implement industrial control systems, including the most common versions of Modbus and Profibus. DH 485 is a proprietary communications protocol used by Allen-Bradley in their line of industrial control units. 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. RS232 uses inverse logic; that is, what is rs485 cable a positive bit at the 68HC11 UART is inverted by the onboard RS232 driver chip and appears as a negative signal on the serial cable. If A is negative with respect to B, the state is binary 1. The reversed polarity (A positive with respect to B) is binary 0. The standard does not assign any logic function to the two states. The terminal’s serial receiver chip re-inverts the signal to its positive sense.

Move the serial cable from the "Serial Port 1" connector to the "Serial Port 2" connector on the QScreen. In this case, cable connections must be made to Serial 1 at pins 5 and 6 of the 10-pin Serial Communications Header. From the QVGA Controller’s point of view, these three signals (TxD, RxD, and ground) are the only connections required to perform serial communications. We can gain insight into the operation of the RS232 protocol by examining the signal connections used for the primary serial port in Table 11 5. The transmit and receive data signals carry the messages being communicated between the QVGA Controller and the PC or terminal. RS-485 signals are used in a wide range of computer and automation systems. 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 68HC11 as a master or slave device. 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.