These protocols are summarized on this page, but for more information regarding their data formats and their use for simplex or multi-drop serial lines, consult Understanding Serial Communications (but keep in mind that that page is directed to the use of the UART Wildcard, so it uses different driver functions). The Serial1 and Serial2 ports have identical communications capabilities, although more of the Serial1 signals (both RS232 and RS485) are made available on the Docking Panels headers and connectors. 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. The GROUND line serves as a common voltage reference for the master and slave. 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 master initiates a communications request to a "slave node" by addressing that unit. RTS (Request to send) does have some utility in certain applications. Because differential signals have inherently better signal-to-noise properties, reliable RS422 communications can be sent over much longer distances compared to RS232. The signal is a voltage change across a load as low as 1.5 volts differential from a driver, with a receiver detecting a differential voltage as minute as 200 millivolts. Receiver Input Resistance (Ohms), (1 Standard Load for RS485) 3k to 7k 4k min. 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). RS232’s greatest benefit is its universality; most personal computers can use this protocol to send and receive serial data. In a finished instrument, either or both channels can be used to communicate with other serial devices, or with other computers and/or terminals using RS232 or RS485. Since both channels can operate simultaneously and independently, serial debugging can be performed while the application program is communicating via its primary channel.

Owing to hardware constraints, if modem handshaking is needed on UART channel 1, then channel 1 must be configured for RS232, and channel 2 cannot be configured for RS232 communications. 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. Can we run the RS485 cable adjacent to or in the same conduit with mains wires? Maximum Cable Length 50 FT. If the total length of the main cable is less than 50 m termination resistances can be avoided at the ends of the main cable. Each serial port can be configured for the RS232 or RS485 protocol, and runs at standard baud rates up to 115,200 bits per second. This chapter describes those drivers, and presents code that makes it easy to configure the SPI for different data transfer rates and formats. Single master systems (many other communications schemes are available) offer a straight forward and simple means of avoiding "data collisions" in a typical 2-wire, half-duplex, multi-drop system.

The PDQ Single Board Computer (SBC) has two asynchronous serial communications ports named Serial1 and Serial2. RS232 is by far the most common serial protocol, and is the default protocol for both of the PDQ Board’s serial ports. The default baud rate after a factory cleanup is 115200 baud. After a factory cleanup, Serial1 is the default serial port for program development and downloading. If your application requires RS485, you can use the secondary serial port (serial2) to program and debug your application code using the RS232 protocol, and use the primary serial port (Serial1) for RS485 communications. A true multi-point network consists of multiple drivers and receivers connected on a single bus, where any node can transmit or receive data. RS423 is another single ended specification with enhanced operation over RS232; however, it has not been widely used in the industry. 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. Electronic data communications between elements will generally fall into two broad categories: single-ended and differential.