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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. 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. The primary serial port, Serial1, is supported by the 68HC11's on-chip hardware UART (sometimes called a USART), and does not require interrupts to work properly. The secondary serial port is implemented by a software UART that controls two pins on PortA. We have built sophisticated instruments using the QScreen Controller that operate very reliably using multiple interrupts in addition to the software UART. Also, several non-serial interrupts can stack up; if they have higher priority than the serial interrupts, they will be serviced before the Serial2 interrupt routine, and again a serial input or output bit may be lost. If an interrupt service routine takes longer than 200 µs, then an entire serial bit will be missed, causing a communications error. The RS485 connections are not brought out to the Serial 1 Connector. Common standards: RS232 RS422 RS449 RS485 20 mA current loop Serial data transmission standards including RS232, RS422, RS423, and RS485 were widely used for many data links, proving effective connectivity for the day.

The Serial2 channel is always configured for RS232 communications, and can sustain baud rates up to 4800 baud. The primary serial channel can operate at standard speeds up to 19200 baud and can be configured for either RS232 (the default) or RS485 operation. For RS232 operation: Remove the jumper shunt from J3. 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. 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! 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. By polling the Port A pin or by setting up an interrupt service routine, you can configure the QScreen to ignore the SCK input when /SS is high and keep MISO in a high-impedance state so that it does not interfere with the SPI bus.

In the middle of a character, each interrupt service routine takes about 35 µs. 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. At the end of a received character, the service routine takes about 45 µs. The DWOM bit (port D wired-or mode) should always be set to 0. Setting DWOM to 1 takes away the processor’s ability to pull the Port D signals high unless there is a pull-up resistor on each bit of the port. Remember that the /SS is active low so to select a device you need to set the pin low; otherwise the pin should idle high. 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. Connecting computers together in multi-drop networks is common in factories and laboratories.

In the most common multi-drop RS-485 protocol, one computer is designated as a "master" and the rest of the computers or devices on the serial bus are designated as "slaves". The /SS (active-low slave select) is typically used to enable data transfers by slave devices when it is active low. When the /SS input goes low, the slave (or QScreen in this case) transfers data in response to the SCK clock input that is initiated by the master. There are a variety of ways the MOSI, MISO, SCK and /SS pins on your QScreen Controller can be connected. RS-485 and RS-422 can interoperate with certain restrictions. RS-485 supports inexpensive local networks and multidrop communications links, using the same differential signaling over twisted pair as RS-422. This would enable the same standards to be used worldwide and also give manufacturers access to a global market using just one product. Serial data communications standards - RS-232, RS-422, & RS-485 were widely used for data transfer providing easy connectivity for the time and they can still be found today. 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.

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