In this project, we have only used a baud rate of 9600 which is well under the maximum transfer speed we can achieve with the MAX-485 Module but this speed is suitable for most of the sensor modules out there and we don’t really need all the maximum speeds while working with Arduino and other development boards unless you are using the cable as an ethernet connection and require all the bandwidth and transfer speed you can get. The specification allows for data transmission from one transmitter to one receiver at relatively slow data rates (up to 20K bits/second) and short distances (up to 50Ft. @ the maximum data rate). Differential data transmission (balanced differential signal) offers superior performance in most applications. RS485 uses the same differential signaling scheme as RS422, and hence has the same superior signal-to-noise characteristics and range described above. RS485 extends the common mode range for both drivers and receivers in the "tri-state" mode and with power off. The "idle" state (MARK) has the signal level negative with respect to common, and the "active" state (SPACE) has the signal level positive with respect to common. Note that the master device outputs the clock synchronization signal SCK to the slave’s SCK which is configured as an input.

The InitSPI() function provides a convenient way to initialize the SPI as the master at a 2MHz baud rate. The dual communications channels also provide an easy way to link systems that communicate using different serial protocols. Two asynchronous communications ports named Serial1 and Serial2 can each be configured for RS232 or RS485 protocols. The PDQ Single Board Computer (SBC) has two asynchronous serial communications ports named Serial1 and Serial2. The secondary serial port is connected similarly except that the onboard connection of RTS to CTS, and DSR to DTR are permanent. EIA-485 (formerly RS-485 or RS485) is an OSI Model physical layer electrical specification of a two-wire, half-duplex, multipoint serial connection. The connection between two or more elements (drivers and receivers) should be considered a transmission line if the rise and/or fall time is less than half the time for the signal to travel from the transmitter to the receiver. Electronic data communications between elements will generally fall into two broad categories: single-ended and differential. If you are using the QScreen as a master device, each external SPI device will require a separate select line (/SS). The RS422 driver and receiver use separate differential conductor pairs on the serial cables, enabling full duplex communications.

While a multi-drop "type" application has many desirable advantages, RS422 devices cannot be used to construct a truly multi-point network. RS422 devices. These networks are often used in a half-duplex mode, where a single master in a system sends a command to one of several "slave" devices on a network. The device that initiates a data transfer is the master, and all other devices on the network are slaves. Typically one device (node) is addressed by the host computer and a response is received from that device. The master initiates a communications request to a "slave node" by addressing that unit. In this manner, data can be exchanged between the master and each slave on the bus. By connecting pairs of these handshaking signals together, the terminal or PC can be made to think that the PDQ Board is always ready to send and receive data. By connecting pairs of these handshaking signals together, the terminal or PC can be made to think that the QScreen Controller is always ready to send and receive data.

RS232’s greatest benefit is its universality; most personal computers can use this protocol to send and receive serial data. When PE is set (equal to one), the most-significant bit in each byte transmitted will be a parity bit that is either set or cleared by the serial port automatically in order to achieve even or odd parity. This 6 ft long cable comes with a USB connector on one end and the red and yellow tipped plugs and does not require a driver if used with Windows 7 or later versions. Two RS485 transceivers are present on the PDQ Board, one for each channel. Of the processor’s three synchronous SPI (Serial Peripheral Interface) ports, two are available for inter-processor communications on multi-processor systems, and the third is brought out to the Wildcard expansion bus. As described below, the RS485Transmit() function controls the RS485 data direction of each serial port. To solve the "data collision" problem often present in multi-drop networks hardware units (converters, repeaters, micro-processor controls) can be constructed to remain in a receive mode until they are ready to transmit data. In general if you are not connected to a modem the handshaking lines can present a lot of problems if not disabled in software or accounted for in the hardware (loop-back or pulled-up).

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