These signals may alternatively be redirected to the digital inputs and outputs used by the second serial port if hardware handshaking is required. For example, at 4800 baud (bits per second), each bit lasts about 200 microseconds (µs), and if communications are full duplex (e.g., if the QScreen Controller echoes each incoming character), then there is a serial interrupt every 100 µs or so. In the middle of a character, each interrupt service routine takes about 35 µs. In most cases the times are less than 25 µs which does not pose a problem. 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. 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. A ground connection is also necessary to ensure that the communicating devices have a common voltage reference. Note that the local and the remote must share a common ground, so a minimum of 3 wires are required for full duplex RS232 communications: a transmit wire, a receive wire, and a common ground.

Note that the local and the remote must share a common ground, so a minimum of 5 wires are required for full duplex RS422 communications: two transmit wires, two receive wires, and a common ground. The RS422 driver and receiver use separate differential conductor pairs on the serial cables, enabling full duplex communications. The Serial Peripheral Interface, SPI, is a fast synchronous serial interface. It provides a convenient means of connecting the QScreen Controller to a variety of peripheral devices, including analog to digital and digital to analog converters, real time clocks, and other computers which use high speed communication. It is a half duplex protocol, meaning that only one party at a time may transmit data. RS485 is a half duplex version of RS422 that is capable of multi-drop operation. This ability to exchange messages means that the SPI is capable of full duplex communication. ANC-6090 converts RS232 level signals to RS422 or RS485 differential signals for noise immune communication at 921 Kbaud at distances up to 4000 feet The adapter can also be used for RS232 to TTL/CMOS serial interfacing. The QScreen Controller’s kernel software contains a complete set of high level driver routines for the Serial2 port, and these functions are summarized in the Control-C Glossary.

Function prototypes for this function and other versatile serial I/O routines are defined in the COMM.H header file, and are described in detail in the Control-C Glossary. If you do this now, remember to move the QScreen Controller’s serial connector back to Serial Port 1, rs485 cable and to change the terminal’s baud rate back to 19200 baud using the "Comm" item under the terminal’s "Settings" menu. 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. The standard C serial I/O routines such as printf(), scanf(), putchar(), and getchar() give you high level access to the serial ports. 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. 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 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! RS485 is another protocol supported by the primary serial port on the QScreen Controller. 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). In this section we will consider the most general and simple configurations. Thus, resetting the SPIF flag is very simple. Then reading the data that was received (by reading the SPDR) or initiating a new data transfer (by writing to the SPDR) automatically clears the SPIF flag. The received data byte is accessed by reading SPDR data register. Any required SPI output signals must be configured as outputs, either by calling InitSPI() or by setting the appropriate bits in the Port D data direction register DDRD. Rather, the UART deduces the correct time to sample the incoming signal based on the start and stop bits in the signal itself.