Also, RS485 drivers are able to withstand "data collisions" (bus contention) problems and bus fault conditions. Also, several non-serial interrupts can stack up; if they have higher priority than the serial interrupts, they will be serviced before the Serial2 interrupt routine, and again a serial input or output bit may be lost. Any number of characters can be sent, and the transmitter will automatically re-trigger with each new character (or in many cases a "bit-oriented" timing scheme is used in conjunction with network biasing for fully automatic operation, including any Baud rate and/or any communications specification, eg. Electronic data communications between elements will generally fall into two broad categories: single-ended and differential. It is NOT necessary to introduce long delays in a network to avoid "data collisions." Because delays are NOT required, networks can be constructed, rs485 cable that will utilize the data communications bandwidth with up to 100% through put. The words port and channel are used interchangeably to refer to a serial communications link.

We recommend that you keep the faster Serial1 port as the default serial link as you work through the exercises in this book. The default baud rate after a factory cleanup is 115200 baud. 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). From the latest point of view, advancements in technology have led to the development of enhanced RS485 cables that can support even higher transmission speeds and longer distances. If your computer does not have an RS232 serial port, low cost USB-to-RS232 serial cables are available; contact Mosaic Industries for details. TXSoil can transmit the humidity to a Web server or to a VisualVision's display / concentrator such as DisplayRX or ControlHUB or RXTXeasy GSM - with adjustable interval, remaining for the rest of the time in standby with very low consumption. 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. A true multi-point network consists of multiple drivers and receivers connected on a single bus, where any node can transmit or receive data.

With the introduction of "automatic" repeaters and high-impedance drivers / receivers this "limitation" can be extended to hundreds (or even thousands) of nodes on a network. RS485 extends the common mode range for both drivers and receivers in the "tri-state" mode and with power off. All of the serial ports are supported by pre-coded C-language software drivers that make it easy to exchange data. RS485 meets the requirements for a truly multi-point communications network, and the standard specifies up to 32 drivers and 32 receivers on a single (2-wire) bus. RS232 has numerous handshaking lines (primarily used with modems), and also specifies a communications protocol. Thus RS485 is the standard protocol of choice when multi-drop communications are required. Chassis and signal grounds are connected together to the digital ground (GND) signal. Differential data transmission (balanced differential signal) offers superior performance in most applications. RTS (Request to send) does have some utility in certain applications. The master initiates a communications request to a "slave node" by addressing that unit.

Independent channels are established for two-way (full-duplex) communications. The following table summarizes the available serial channels. The PDQ Single Board Computer (SBC) has two asynchronous serial communications ports named Serial1 and Serial2. Two asynchronous communications ports named Serial1 and Serial2 can each be configured for RS232 or RS485 protocols. RS422 (differential) was designed for greater distances and higher Baud rates than RS232. It is designed to transmit data over long distances and in noisy environments. With noisy environments (our sites?) using the shield as ground is not always the most effective solution. Systems of this type (4-wire, half-duplex) are often constructed to avoid "data collision" (bus contention) problems on a multi-drop network (more about solving this problem on a two-wire network in a moment). 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).