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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. Chassis and signal grounds are connected together to the digital ground (GND) signal. The RS232 protocol specifies the use of two separate grounds, a signal ground and a protective (or "chassis") ground. From the QScreen Controller’s point of view, these three signals (TxD, RxD, and ground) are the only connections required to perform serial communications. Its simplest implementation requires only three wires: one to transmit serial data, a second to receive serial data, and a third to provide a common ground reference. 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 BufferToSPI() function implements fast data transfer from a specified buffer in the controller’s memory to an SPI device.

You decide that the water is not filling fast enough to keep up with demand. All of this allows you to speed up the VFD that controls the speed of that water pump. It controls the serial-to-parallel and parallel-to-serial conversion and performs all of the timing functions necessary for asynchronous serial communications. The Serial 1 port can be configured for either RS232 or RS485 communications at up to 19200 baud. Data translation between different machines can be performed with ease, and applications that communicate via the primary serial port can be debugged using the secondary channel. 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 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 channel’s UART translates the bit-by-bit data on the serial cable into bytes of data that can be interpreted by the QED-Forth Kernel or by your application program. The wide supply voltage range also means that you can allow for a significant voltage drop along the bus (e.g., when using a bus cable of considerable length).

5 Volt supply by a dual RS232 driver chip that has a built-in charge pump voltage multiplier. The GROUND line serves as a common voltage reference for the master and slave. 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 QScreen Controller combines an embedded computer based on the 68HC11 microcontroller with a touch panel and LCD (liquid crystal display) graphic user interface (GUI) that is ideal for instrument control and automation. A simple network of a PLC, VFD, and an HMI allows remote control of motors in an industrial setting. Flexible and Simple application. Thus, resetting the SPIF flag is very simple. After a data transfer is initiated by writing to the SPDR data register, the processor may poll the SPSR status register until the SPIF flag is set. The SPIF is set when a data transfer is complete, and is cleared by a read of the SPSR status register, followed by a read or write to the SPDR data register. The WCOL flag is set when a write collision occurs.

The cable to be used is a shielded twisted pair (telephone type). Unlike the standard RS232 protocol, RS485 allows many communicating parties to share the same 3-wire communications cable. A hardware reset (pressing down on the reset switch) has the same effect. 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. 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. RS232 allows both communicating parties to transmit and receive data at the same time; this is referred to as full duplex communications. 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! Typing a carriage return at the terminal should now produce the familiar "ok" response via the Serial2 port. Now select the "Comm" item in the "Settings" menu of the Terminal program, and click on 1200 baud (or whatever baud rate you selected in the command above).

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