Once the Bytes have been Exchanged

The SCK pin clocks the serial A/D’s CLK input which causes the A/D’s conversion result to be transferred to the master via the MISO line. Failure to stay within this range will result in, at best, signal corruption, and, at worst, damage to connected devices. In this section we will consider the most general and simple configurations. 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 secondary serial port is implemented by a software UART that controls two pins on PortA. 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.

The RS485 protocol uses differential data signals for improved noise immunity; thus RS485 can communicate over greater distances than RS232. Since both channels can operate simultaneously and independently, debugging can be performed while the application program is communicating via its primary channel. A jumper, J3, configures the primary serial port for either RS232 or RS485 operation. 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. Our USB development kits use GasLab® software for setup, calibration, data logging, and real-time data analysis. 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. Rather, it relies on software handshaking via transmission of XON/XOFF characters to coordinate data transfer and ensure that information is not lost when one of the communicating parties is busy. These detailed signal descriptions and cable diagrams are presented to provide complete information for those who have special communications requirements and for those who wish to make their own application-specific communications cables.

The communications is asynchronous because no synchronizing clock signal is transmitted along with the data. RS485 cable is a type of communication cable commonly used in industrial settings to transmit data between devices. The transmission speed of RS485 cables can vary depending on factors such as cable length, baud rate, and the quality of the cable itself. You can use it to communicate with other devices. Because all of the serial I/O routines on the QScreen Controller are revectorable, it is very easy to change the serial port in use without modifying any high level code. C program’s source code file. 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. 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. The RS232 protocol specifies the use of two separate grounds, a signal ground and a protective (or "chassis") ground. 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.

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. 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. You can operate the port at any baud rate up to 4800 baud; just specify the rate you want before the BAUD2 command. For those of you interested in the details, here’s how it works: The low-level serial driver routines named Key(), AskKey() and Emit() are revectorable routines that can be redirected to use either of the serial ports. 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.