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RS232 uses inverse logic; that is, 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. Thus in Table 9 6 , RTS1 is connected to CTS1, and DSR1 is connected to DTR1 and DCD1 onboard the QScreen Controller using zero ohm shorting resistors. There are a variety of ways the MOSI, MISO, SCK and /SS pins on your QScreen Controller can be connected. 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. 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.

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 status of a device as master or slave determines how the various pins must be configured. The secondary serial port is implemented by a software UART that controls two pins on PortA. In fact, the program works the same as it did before, but now it is using the secondary serial port instead of the primary port -- and you didn’t even have to recompile the code! The secondary channel is very useful for debugging application programs that communicate with other computers or I/O via the primary channel. Since both channels can operate simultaneously and independently, debugging can be performed while the application program is communicating via its primary channel. You can operate the port at any baud rate up to 4800 baud; just specify the rate you want before the BAUD2 command. If you have not yet compiled the GETSTART program and you want to do the exercises here, open GETSTART.C in your TextPad editor, click on the Make Tool, and after the compilation is done, enter Mosaic Terminal by clicking on the terminal icon and use the "Send File" menu item to send GETSTART.DLF to the QScreen Controller.

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. The RS485 connections are not brought out to the Serial 1 Connector. From the QScreen Controller’s point of view, these three signals (TxD, RxD, and ground) are the only connections required to perform serial communications. These signals may alternatively be redirected to the digital inputs and outputs used by the second serial port if hardware handshaking is required. 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. Typing a carriage return at the terminal should now produce the familiar "ok" response via the Serial2 port.

The PDQ Board controls the Serial1 and Serial2 RS485 transceivers with bits PJ0 and PJ1, respectively, of PORTJ of the processor. 1 or 2 to specify Serial1 or Serial2, respectively, and clears the appropriate PORTJ bit to place the transceiver in receive mode. We recommend that you keep the faster Serial1 port as the default serial link as you work through the exercises in this book. It is supported by virtually all personal computers, and is the default protocol for both of the QScreen Controller’s serial ports. 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. Given a properly wired network and a properly configured SPCR control register, a master device may transmit a message by simply storing the byte to the SPDR data register. A data transfer is initiated by a master device when it stores a message byte into its SPDR register. While these signals provide a data path, they do not provide hardware handshaking that allows the two communicating parties to let each other know when they are ready to send or receive data.

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