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In this case, cable connections must be made to Serial 1 at pins 5 and 6 of the 10-pin Serial Header or pins 7 and 8 on the 24-pin Field Header. In this case, cable connections may be made to Serial 1 on either the 10-pin Serial Communications Header or the Serial 1 Connector. 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 68HC11 allows the details of the synchronous communications protocol to be customized for compatibility with a variety of peripherals. 3. Could you do customized functions according to requirements ? 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), what is rs485 cable and DGND (digital ground). However, note that the functions that write to EEPROM disable interrupts for 20 msec. 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.

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. 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 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. RS232’s greatest benefit is its universality; practically all personal computers can use this protocol to send and receive serial data. You can use the QScreen’s RS485 link to create such a multi-drop serial network. RS485 is another protocol supported by the primary serial port on the QScreen Controller. The RS485 protocol uses differential data signals for improved noise immunity; thus RS485 can communicate over greater distances than RS232. 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. To reduce the effect of interference from external electrical signals the RS485 interface uses a differential signal over a "twisted pair" cable.

A hardware reset (pressing down on the reset switch) has the same effect. 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. To interface devices that support synchronized serial interfaces, but are not configurable like the 68HC11, determine the device’s requirements for clock phase and polarity and configure the 68HC11’s CPHA and CPOL accordingly. The flexibility and power of the 68HC11’s serial peripheral interface supports high speed communication between the 68HC11 and other synchronous serial devices. The only difference between the master and slave devices is that the master initiates the transmission. Slave devices use the master in/slave out pin, MISO, for transmitting, and the master out/slave in pin, MOSI, for receiving data. 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). 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.

The two lowest order bits in the SPCR control register, named SPR1 and SPR0, determine the data exchange frequency expressed in bits per second; this frequency is also known as the baud rate. The secondary serial port is implemented by a software UART that controls two pins on PortA. 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. 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. The Serial 2 port is dedicated to RS232 communications at up to 4800 baud. The maximum Serial2 communications rate is 4800 baud. RS232 allows both communicating parties to transmit and receive data at the same time; this is referred to as full duplex communications. Connecting a standard full duplex link RS232 between two computers is the same as with a standard RS232 link, with the TxD (transmitter output) of each computer connected to the RxD (receiver input) of the other computer. 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.