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Serial 2 is implemented by a software UART in the controller’s QED-Forth Kernel that uses two of the processor’s PortA I/O pins to generate a serial communications channel. The DMX standard uses that RS485 (or, for clarity RS422 would be better) standard for its hardware medium description, but that's where it ends. One transmitter and 10 receivers are allowed on a network, with a distance limit of 3600ft. RS422 was on the market prior to RS485; however, due to loading limitations, one of the best uses of RS422 is probably in point-to-point communications, such as RS232 extension cords. Here again, we start with including the standard library for driving the LCD and declare the D8 pin of the Arduino Nano as an output pin which we will later use to declare the MAX485 Module as a transmitter or Receiver. We will pull the enable pin high to put the MAX485 module in transmitter mode. The receiver load impedence is 12K ohms (or higher) and transmitter "leakage" current is ±100µA (or less) in either the powered or unpowered state.

5V power supply and the rated current is 300 μA. They are often used for serial connections going to a terminal server in datacentres, for example. 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). If your computer does not have an RS-232 serial port, low cost USB-to-RS-232 serial cables are available; contact Mosaic Industries for details. The RS485 system used for Modbus communication provides a main cable (Bus or backbone), to which all the devices have to be connected with branches (also known as stubs) that are as short as possible. This allows RS-485 to implement linear bus topologies using only two wires.

If you do this now, remember to move the QVGA Controller’s serial connector back to Serial Port 1, and to change the terminal’s baud rate back to 9600 baud using the "Communications" item under the terminal’s "Settings" menu. The InitSPI() function provides a convenient way to initialize the SPI as the master at a 2MHz baud rate. 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. Data types are mapped using standard addresses. The shielding may be braided (be formed by a mesh of thin conducting wires) or be a foil (consisting of a sheet of metal wound around the conductors): the two types are equivalent. Since Modbus was designed in the late 1970s to communicate to programmable logic controllers, the number of data types is limited to those understood by PLCs at the time. This section also defines the logic states 1 (off) and 0 (on), by the polarity between A and B terminals.

All the ‘A’ terminals must be connected together and all the ‘B’ terminals must be connected together respectively. The main cable must be no longer than 700 m! The branches must be no longer than 1200 m! Longer branches could cause signal reflections and generate disturbances and consequent errors in the reception of data. 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. One reason is that, large networks accumulate distributed electrical noise which can make communications unreliable. To make sure that your USB to RS-485 cable & the device RS-485 port are working, have you tried using them with a Windows PC and the manufacturer's s/w? The DWOM bit (port D wired-or mode) should always be set to 0. Setting DWOM to 1 takes away the processor’s ability to pull the Port D signals high unless there is a pull-up resistor on each bit of the port.

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