The RS232 signals are represented by voltage levels with respect to a system common (power / logic ground). RS422 (differential) was designed for greater distances and higher Baud rates than RS232. 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, RS485 standard eg. 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. RS485 will support 32 drivers and 32 receivers (we are talking about bi-directional - half duplex - multi-drop communications over a single or dual twisted pair cable !!). 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. 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.

This is a example of point to point (one port communicates with one device). In short, the RS232 port was designed to communicate with local devices, and will support one driver and one receiver. One pair of twisted pair conductors is used for the RS-485 half duplex communication, while the other pair on the output only has power for peripheral devices, such as a keyboard and display panel, isolated input sensors, and isolated output drivers. 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. RS-485 only specifies the electrical characteristics of the generator and the receiver: the physical layer. RS232 has numerous handshaking lines (primarily used with modems), and also specifies a communications protocol. The master initiates a communications request to a "slave node" by addressing that unit. One RS-485 network can be used for the master to talk to all of the slaves on the network, and a different RS-485 network can be used for all of the slaves to talk back to the master. 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).

This allows RS-485 to implement linear bus topologies using only two wires. RS-232 allows for both a transmit and receive circuit. As a general rule when the voltage measured (with respect to circuit common) is "high" the binary state is considered to be a 1 and when the voltage is low the state is 0. There are exceptions to this rule (such as differential logic and negative logic), but for the purpose of this discussion we'll talk about normal logic. 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). 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. RS422. RS422 is also specified for multi-drop (party-line) applications where only one driver is connected to, and transmits on, a "bus" of up to 10 receivers.

In the RS-485 standard, one wire is labeled "A" and the other is labeled "B", and the wires are twisted together (a "twisted pair"). Typical UART communications with an RS-422 device, uses both a transmitter and receiver on a network of two balanced pairs of wires for a total of four wires. The RS-422 standard title is "Electrical Characteristics of Balanced Voltage Digital Interface Circuits". There is no hardware handshaking in the RS-485 standard and in most cases it is no longer needed. Once a requested data is sent the hardware reverts back into a receive mode. An RS-485 network can be connected in a 2 or 4 wire mode. But biasing is discussed as an internal receiver characteristic, not an external biasing network. This figure shows biasing applied to the termination network. 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). While a multi-drop "type" application has many desirable advantages, RS422 devices cannot be used to construct a truly multi-point network. This type of signal works well in point to point communications at low data transmission rates.