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It defines a means of exchanging control signals and binary serialized data signals between data processing terminal equipment and data communication equipment, and is of particular importance when each is furnished by a different company. It defines a means of exchanging control signals and binary serialized data signals between data terminal equipment and a data communication channel in cases where interchange point. PURPOSE AND SCOPE OF COVERAGE - This standard is intended to provide a method of interconnecting data terminal equipment and a data communication channel when each is furnished by different companies. This type of port was needed to transfer data from/to a printer, modem, or in rare cases a mouse. Converters that changed the RS-232 electrical levels to RS-485 levels were developed which allow the serial port to transfer data over much longer distances than the RS-232 levels allowed. PURPOSE AND SCOPE OF COVERAGE - This standard is applicable to the interconnection of data terminal equipment (DTE) and data communication equipment (DCE) employing serial binary data interchange. PURPOSE AND SCOPE OF COVERAGE - This standard is applicable to the interconnection of data processing terminal equipment and data communication equipment.

Implementing this requires a higher level addressing scheme so the data being transmitted goes to the correct device. The interface signal level is lower than RS232-C, it is not easy to damage the interface circuit chip, and the level is compatible with TTL level, which is convenient for connection with TTL circuit. The RS-232 standard does not include a line length limit, but practical limits of the electrical signal levels prevent long lengths. In summary, the EIA/TIA standards (which are commonly miscalled RS standards) are the physical layer which include electrical signal levels and RS-232 includes the connector. When IBM introduced the PC/AT they also changed the physical format of the serial port connector to a D-sub, E-shell, 9-pin connector (a DE-9 commonly miscalled a DB-9). The serial port used a D-sub, B-shell, 25-pin connector (commonly called a DB-25) and RS-232 electrical signal levels, which made it "RS-232 compatible" or compatible with the RS-232 standard.

The new IBM PC/AT serial port no longer used the RS-232 25-pin connector or signal levels, but IBM took care to ensure that the new serial port’s electrical levels would still work with existing serial port peripherals, requiring only a 9-pin to 25-pin adapter. But, since existing RS-232 compatible peripherals still functioned with it, it was still regularly called an "RS-232 port". While this is incorrect, many to this day still believe that the RS-232 standard includes things such as baud rate and the bit stream protocol used in the IBM serial port. It is important to understand that all four serial interface standards allow different baud or data-transmission rates. This was done since this protocol is a very common bit format used with serial communications. The original "RS-232 compatible" serial port used a UART to drive the RS-232 electrical signal levels and many came to believe that the UART protocol was a part of the RS-232 standard. The drive capacity of RS485 is 32-unit loads, equivalent to 375Ω. The maximum recommended data rate for RS485 was initially 10Mbps, but over the years, the standard has evolved to allow for higher data rates and reliable communication over long distances.

The original PC was upgraded with a hard drive and became the PC/XT. Later it was upgraded again to a 80286 processor and a 16-bit EISA bus and became the PC/AT. The RS422/RS485 bus needs to be terminated with the resistor on both ends (so-called parallel termination), which is equal to the characteristic impedance of the used cable, to prevent line reflections. A 120-ohm resistor should be used to terminate the receiving end of the line. There are two other devices in this figure (receiver and transciever) that are connected to the transmission line the same way. Many references and examples are made to "asynchronous start-stop communication with a UART" or "UART" communications. 4.RS485 can connect up to 32 nodes on the same bus, which can realize real multi-point communication, but generally adopts the master-slave communication mode, that is, one master has multiple slaves. Loaded driver voltage (32 nodes on the network and termination) should typically exceed ±1.5V.

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