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RS-485 is used for low-speed data communications in commercial aircraft cabins' vehicle bus. This chapter describes those drivers, and presents code that makes it easy to configure the SPI for different data transfer rates and formats. Confirm serial port parameters such as baud rate, data bits, and stop bits: Baud rate should be configured according to the connected RS485 device, commonly used rates are 9600 and 115200; Refer to the device manual or contact RS485 device technical support for configuration, especially if set to Modbus mode, confirm if the function code matches. Some of these converters can convert from RS232 to both RS485 and RS422 however many standard models only has one of the protocols. You can implement the slave select lines by configuring Port A pins as outputs. The maximum sustainable baud rate on the secondary serial port is 4800 baud. The InitSPI() function provides a convenient way to initialize the SPI as the master at a 2MHz baud rate. 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. 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.

The byte-sized messages are transmitted and received via the MOSI (master out/slave in) and MISO (master in/slave out) pins. Configured as a master device, the QScreen transmits bytes via the "master out/slave in" pin, MOSI. Transmissions are always initiated by the master device, and consist of an exchange of bytes. 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. Slave devices use the master in/slave out pin, MISO, for transmitting, and the master out/slave in pin, MOSI, for receiving data. The RS485 converter interface uses two wires for communication, one for transmitting and one for receiving data, which is referred to as a half-duplex communication converter. 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. If you are using the QScreen as a master device, each external SPI device will require a separate select line (/SS). 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. As the master transmits a byte to an active slave (that is, a slave with its /SS input active low), the master receives a byte from the slave. A Modbus RS485 connects a Master device to one or more Slave devices. If the PC is connected to the multiple Laureate instruments on an RS485 bus via an Ethernet-to-RS-485 gateway board or a USB-to-RS485 gateway board, use Laurel’s free XLOG2 datalogging software. A challenge is that Laureate meters use RJ11 jacks, while Laureate LT series transmitters use screw-clamp connectors for three discrete wires.

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). 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. All the ‘A’ terminals must be connected together and all the ‘B’ terminals must be connected together respectively. In order to avoid errors when many devices are connected, cables of the same colour should be used for all the connections to the terminals A and cables of the same colour should be used for all the connections to the terminals B of the various devices (e.g. white for A and blue for B). Test options include: audio, video, ping, TDR, PoE, IP network connections, and LAN (local area network) cable connections. Custom FTDI USB - RS485 Cable with Molex connector, fits all GasLab EC and MX Smart Sensor Boards and Development Kits. In order to avoid signal reflections, a 120 Ohm termination resistance must be fitted on each end of the main cable. This distance does not include the branches (which must nevertheless be short).

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