The subsequent 3 Things To immediately Do About Rs485 Cable > 자유게시판

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 master and slave can then exchange data. To ensure that no two devices drive the network at the same time, it is necessary that each slave device be able to disable it’s own RS-485 data transmitter. In theatre and performance venues, RS-485 networks are used to control lighting and other systems using the DMX512 protocol. The RS485 interface allows the creation of multipoint networks. Each of the two UARTs on the wildcard is capable of full-duplex communications, meaning that both transmission and reception can occur simultaneously (although the RS485 protocol is half duplex as explained below). Although the maximum standard baud rate of the primary serial port is 19200 baud, nonstandard baud rates of over 80 Kbaud can be attained by the 68HC11's on-chip UART and the onboard RS232 driver. The maximum sustainable baud rate on the secondary serial port is 4800 baud. The SCK (serial clock) pin is a configurable synchronous data clock output.

InitRS485() configures Port D to ensure that bit 5 is an output. 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 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. 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. Also, in the diagram, the master’s /SS (slave select) is configured as an output. 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. For the QScreen, /SS is not used for SPI communication because it is used to control the direction of the RS485 transceiver; you can use any digital I/O line as a /SS signal.

With Modbus, BACnet and Profibus, A/B labeling refers A as the negative green wire and B as the positive red wire, in the definition of the D-sub connector and M12 circular connector, as can be seen in Profibus guides. 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. If the programmer has enabled the local interrupt mask for the SPI, an interrupt is recognized at this point. Alternatively, the if the SPI interrupts are enabled, the SPI interrupt handler determines what caused the interrupt by reading the SPSR register to see which of the three status bits is set. The SPIE bit in the SPCR (SPI control register) enables SPI interrupt handling. Moreover, if Serial2 is running full duplex at 4800 baud, any other interrupt service routine that takes longer than 100 µs is likely to cause a problem.

Because the software UART is interrupt based, competing interrupts that prevent timely servicing of the Serial2 interrupts can cause communications errors on the secondary serial channel. The SPI can transfer data much more rapidly than an asynchronous serial link - its maximum rate is 2 Megabits/second. It does not specify or recommend any communications protocol; Other standards define the protocols for communication over an RS-485 link. You can use the QScreen’s RS485 link to create such a multi-drop serial network. The Serial2 channel is always configured for RS232 communications, and can sustain baud rates up to 4800 baud. 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. Routines that temporarily disable interrupts for significant periods of time can also interfere with the Serial2 port. When the exchange is complete, the slave can again execute the Silence() routine to disable its transmitter and begin listening for its name. It receives bytes sent by a slave device via the "master in/slave out" pin, MISO.

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