Here's What I Know About Rs485 Cable > 자유게시판

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QED-Forth includes three built-in routines to facilitate control of the RS485 transceiver. Hardware is interfaced to the SPI via three PORTD pins named SCK, MOSI, and MISO brought out to pins 7, 8, and 10 on the Wildcard Port Header (see Appendix B). Regardless of the network, however, there are only four signals used: SCK provides a synchronized clock, MOSI and MISO signals are used for data transmission and reception, and /SS configures the QScreen as a master or slave device. When the QScreen controls the network, it is referred to as a "master"; otherwise, it is a "slave". In order to increase the extent of the Modbus network, repeaters can be used; and signal amplifying and regenerating devices provided with two communication ports that transfer to each what they receive from the other. 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.

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. 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. 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 are running Serial2 at 4800 baud, the rest of your application must be able to function properly using the remaining portion of the CPU time. The InitSPI() function provides a convenient way to initialize the SPI as the master at a 2MHz baud rate. Thus, the master has only one input, MISO, which is the slave’s only output. Also, in the diagram, the master’s /SS (slave select) is configured as an output. 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.

The data exchange format may be a line of ascii text. This automatically activates the SCK clock which synchronously transmits the data. This setting is only relevant for the master device, as it is the master’s clock which drives the transfer. Even though the MOSI pin is not connected to anything, the master initiates a transmission using a "dummy" byte. Once the bytes have been exchanged, the master may write a new byte to initiate another byte exchange. The received data byte is accessed by reading SPDR data register. After configuring the SPI system to communicate on a properly connected network of devices, sending and receiving data is as simple as writing and reading a register. In this section we will consider the most general and simple configurations. The one you choose depends on the specific device, or devices you will be connecting to. 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. In these distributed processing networks, a variety of machines and instruments work locally, rs485 cable but communicate and share data or resources with one another globally using a single serial link.

The QScreen allows the details of the synchronous communications protocol to be customized for compatibility with a variety of peripherals. 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. 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. This detects the presence of more than one master on the SPI bus. The SPI can transfer data much more rapidly than an asynchronous serial link - its maximum rate is 2 Megabits/second. This way, the lines will be biased to known voltages and nodes will not interpret the noise from undriven lines as actual data; without biasing resistors, the data lines float in such a way that electrical noise sensitivity is greatest when all device stations are silent or unpowered. If you are using the QScreen as a slave device and require the /SS signal for your external SPI hardware, configure one of the Port A pins on the Field Header as an input pin.