4 Ways To Simplify Rs485 Cable > 자유게시판

When the network master wants to talk to this particular slave, it outputs the slave’s ascii name onto the serial bus. Note that the master device outputs the clock synchronization signal SCK to the slave’s SCK which is configured as an input. Note that the data is changed by the transmitting device one half clock cycle before it is valid. 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. If you are using the QScreen as a master device, each external SPI device will require a separate select line (/SS). 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. 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. 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.

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. 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. Finally, for master devices, the SPR1 and SPR0 bits determine the baud rate at which data is exchanged. You might also consider operating the secondary serial port at a lower baud rate to relax the timing constraints. 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 maximum sustainable baud rate on the secondary serial port is 4800 baud. The foreword to the standard references The Telecommunications Systems Bulletin TSB-89 which contains application guidelines, including data signaling rate vs. The rate of data transmission is expressed in bits per second, or baud.

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. 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". This section describes the QED-Forth routines that control the RS485 transceiver, and presents some ideas that may prove useful in designing a multi-drop data exchange protocol. Because the requirements of every multi-drop application are so unique, it is difficult to specify or design a software protocol that meets everyone’s needs. RS485 is in its design a different interface from RS232. For communications, many of their products are equipped with an RS232 or RS485 serial port. Bus Probes are very compact (measuring only 40x36mm externally) and sport an attractive look and feel. This allows RS-485 to implement linear bus topologies using only two wires.

Two Tibbo Project System (TPS), Gen. 2-compatible Tibbo BASIC applications for the setup and testing of Bus Probes are available: BP-Tester-UI and BP-Tester-Web. When the keyword name is received by the Silence() routine running in the slave, the slave QScreen Controller executes RS485Transmit() to send an acknowledgment to the master (which should now be listening to the serial bus to accept the acknowledgment). It receives bytes sent by a slave device via the "master in/slave out" pin, MISO. It may be that only the byte sent from the master to the slave is meaningful; nevertheless, each device simultaneously transmits and receives one byte. Even though the MOSI pin is not connected to anything, the master initiates a transmission using a "dummy" byte. The only difference between the master and slave devices is that the master initiates the transmission. The device that initiates a data transfer is the master, and all other devices on the network are slaves. The remaining "inactive" slaves may actively receive, or listen to, data on the communications line, but only one slave at a time can transmit a message.

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