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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. Because a single pair of conductors is used for both transmission and reception, RS485 is useful for multi-drop applications in which a master communicates with multiple slave serial devices, or nodes. A UART is a Universal Asynchronous Receiver/Transmitter that converts parallel data from the host processor (any Mosaic controller) into a serial data stream. 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). 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. With careful design, many peripherals can communicate via the SPI, and powerful multi-processor systems can be linked using this high speed bus. At the UART, a logic high (5 volts) is called a mark, and a logic low (0 volts) is called a space. When communicating at high data rates, or over long distances in real world environments, single-ended methods are often inadequate.

Rather, it relies on software handshaking via transmission of XON/XOFF characters to coordinate data transfer and ensure that information is not lost when one of the communicating parties is busy. Differential data transmission (balanced differential signal) offers superior performance in most applications. To ensure that no two devices drive the network at the same time, it is necessary that each slave device be able to disable it’s own RS-485 data transmitter. Note that the master device outputs the clock synchronization signal SCK to the slave’s SCK which is configured as an input. This setting is only relevant for the master device, as it is the master’s clock which drives the transfer. Setting SPE (SPI enable) to 1 turns on the SPI system. The RS232 signals are represented by voltage levels with respect to a system common (power / logic ground). These factors are discussed in more detail in The UART Wildcard: Modem Handshaking Signals. 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.

It is important to note that when the CPHA bit is 0, the /SS line must be de-asserted and re-asserted between each successive data byte exchange (68HC11 Reference Manual, Section 8.3.2). If the CPHA bit is 1, the /SS line may be tied low between successive transfers. Electronic data communications between elements will generally fall into two broad categories: single-ended and differential. 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. The hardware detects the start-bit of the transmission and automatically enables (on the fly) the RS485 transmitter. 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). It does not specify or recommend any data protocol. RS232 has numerous handshaking lines (primarily used with modems), and also specifies a communications protocol. Most modems communicate using RS232 and a set of hardware handshaking signals used to regulate data flow. The difference between the wires’ voltages is what conveys the data.

Differential signals can help nullify the effects of ground shifts and induced noise signals that can appear as common mode voltages on a network. A modem (modulator/demodulator) provides a way of encoding digital data as a set of audio signals that can be sent over a telephone line. It is NOT necessary to introduce long delays in a network to avoid "data collisions." Because delays are NOT required, networks can be constructed, that will utilize the data communications bandwidth with up to 100% through put. The one you choose depends on the specific device, or devices you will be connecting to. In the most common multi-drop RS-485 protocol, one computer is designated as a "master" and the rest of the computers or devices on the serial bus are designated as "slaves". The local and remote must share a common ground, so all serial cables include at least one ground conductor. Although the devices would share the same network, communications would only be understandable by members of the same group. Note that the local and the remote must share a common ground, so a minimum of 3 wires are required for full duplex RS232 communications: a transmit wire, a receive wire, and a common ground.

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