When the /SS Input Goes Low > 자유게시판

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. If the /SS input to a slave is inactive (high), the slave ignores the SCK input, does not transmit or receive data, and keeps its MISO output in a high-impedance state so that it does not interfere with the SPI bus. 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. The SPI control register, SPCR, contains 8 bits which must be initialized for proper control of the QVGA Controller’s SPI (M68HC11 Reference Manual, what is rs485 cable p.8-7). It is important to note that when the CPHA bit is 0, the /SS line must be de-asserted and re-asserted between each successive data byte exchange (M68HC11 Reference Manual, p.8-3). A write collision occurs when a byte is written to the SPI data register, SPDR, while data is being exchanged.

If bit 5 of DDRD is 1, then /SS is a general purpose output that operates independently of the SPI. If the 68HC11 is initialized as a master (by setting the MSTR bit in the SPCR control register as explained below) then bit 5 of the Port D data direction register (DDRD) determines whether /SS is an input or an output. If bit 5 of DDRD is 0, then /SS is an input. If the /SS input to a slave is active (low), the slave transfers data in response to the SCK clock input that is initiated by the master. The /SS (active-low slave select) signal enables data transfers by slave devices when it is active low. In general, all devices on a network should use the same phase, polarity, and baud rate clock signal. You can use the QED Board’s RS485 link to create such a multi-drop serial network.

This allows standard point-to-point full duplex communications, as well as a multi-drop configuration with one master (a single QVGA Controller or a desktop computer) and multiple QVGA Controller slaves. This feature supports joining monitored data from multiple serial devices into a single monitoring session. Modbus Serial Line protocol is a master-slave protocol which supports one master and multiple slaves in the serial bus. The QScreen Controller has two serial communications ports: a primary serial port called Serial 1 that supports both RS232 and RS485 protocols, and a secondary serial port called Serial 2 that supports RS232. Connecting a standard full duplex link RS232 between two computers is the same as with a standard RS232 link, with the TxD (transmitter output) of each computer connected to the RxD (receiver input) of the other computer. In some countries, inserting two cables into the same screw terminal is permitted. Unlike the standard RS232 protocol, RS485 allows many communicating parties to share the same 3-wire communications cable.

The software routines, RS232Transmit() and RS232Silent() control the dual RS232 transmitters on the board. To properly operate the network each slave computer executes RS232Silent at startup; thus all of the slave transmitters remain silent individually addressed by the master. When the communication is complete the slave silences its transmitter so that the master can talk to a different slave on the network. Serial port monitoring and data processing algorithms are extremely optimized in order to provide you with slick user experience and eliminate all negative impact on operating system performance, stability, computer resources consumption, and introduction of communication delays even at high non-standard data transfer rates. 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. If your computer does not have an RS-232 serial port, low cost USB-to-RS-232 serial cables are available; contact Mosaic Industries for details.