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A data transfer is initiated by a master device when it stores a message byte into its SPDR register. Given a properly wired network and a properly configured SPCR control register, a master device may transmit a message by simply storing the byte to the SPDR data register. Unique rapid auto data direction control reverses a half-duplex channel without software or hardware intervention. The primary serial channel can operate at standard speeds up to 19200 baud and can be configured for either RS232 (the default) or RS485 operation. You can use the QScreen’s RS485 link to create such a multi-drop serial network. 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. Pre-coded device drivers configure the SPI for a standard data format, and it is easy to customize a data format and baud rate for your application.

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. 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. The SPIE bit in the SPCR (SPI control register) enables SPI interrupt handling. Any of these conditions may generate an interrupt if the SPIE (SPI interrupt enable) bit in the SPCR control register is set. Only one active master may control the network at a time; however, the device that assumes the role of master may change according to an appropriate protocol. The one you choose depends on the specific device, or devices you will be connecting to. If you need technical support or have any feedback/review, please click the Submit Now button to submit a ticket, Our support team will check and reply to you within 1 to 2 working days. If you are using the QScreen as a master device, each external SPI device will require a separate select line (/SS). The /SS (active-low slave select) is typically used to enable data transfers by slave devices when it is active low.

This function properly configures the directions of the SPI I/O pins, and configures the data transfer such that data is valid on the falling trailing edge of the clock, with the clock idling in the low state. InitRS485() configures Port D to ensure that bit 5 is an output. The maximum sustainable baud rate on the secondary serial port is 4800 baud. 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. We have built sophisticated instruments using the QScreen Controller that operate very reliably using multiple interrupts in addition to the software UART. The Control-C Glossary contains a list of functions that temporarily disable interrupts, and the glossary entries give further information regarding how long interrupts are disabled. However, note that the functions that write to EEPROM disable interrupts for 20 msec.

Note that the master device outputs the clock synchronization signal SCK to the slave’s SCK which is configured as an input. By setting this output LOW, the slave’s input /SS is pulled LOW. Thus, the master has only one input, MISO, which is the slave’s only output. 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. A RS485 bus consists of one master and multiple slaves. 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". Not more than one breakout board is required per RS485 line. You can connect the board to your host device using a standard mini USB cable. Using a repeater, the main cable is divided into different segments, each of which can be up to 700 m in length and connect 32 devices (this number includes the repeaters).

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