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[ pdf ] RS232 Fundamentals

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Fundamentals of RS–232 · Serial Communications · APPLICATION NOTE 83 · 030998 1/9 · Due to it's relative simplicity and low

Summary:
RS–232 SPECIFICATIONS
RS–232 is a “complete” standard. This means that the standard sets out to ensure compatibility between the host and peripheral systems by specifying 1) common voltage and signal levels, 2)common pin wiring configurations, and 3) a minimal amount of control information between the host and peripheral systems. Unlike many standards which simply specify the electrical characteristics of a given interface, RS–232 specifies electrical, functional, and mechanical characteristics in order to meet the above three criteria. Each of these aspects of the RS–232 standard is discussed below.
ELECTRICAL CHARACTERISTICS
The electrical characteristics section of the RS–232 standard includes specifications on voltage levels, rate of change of signal levels, and line impedance.
The original RS–232 standard was defined in 1962. As this was before the days of TTL logic, it should not be surprising that the standard does not use 5 volt and ground logic levels. Instead, a high level for the driver output is defined as being +5 to +15 volts and a low level for the driver output is defined as being between –5 and –15 volts. The receiver logic levels were defined to provide a 2 volt noise margin. As such, a high level for the receiver is defined as +3 to +15 volts and a low level is –3 to –15 volts. Figure 1 illustrates the logic levels defined by the RS–232 standard. It is necessary to note that, for RS–232 communication, a low level (–3 to –15 volts) is defined as a logic 1 and is historically referred to as “marking”. Likewise a high level (+3 to +15 volts) is defined as a logic 0 and is referred to as “spacing”.
The RS–232 standard also limits the maximum slew rate at the driver output. This limitation was included to help reduce the likelihood of cross–talk between adjacent signals. The slower the rise and fall time, the smaller the chance of cross talk. With this in mind, the maximum slew rate allowed is 30 V/ms. Additionally, a maximum data rate of 20k bits/second has been defined by the standard. Again with the purpose of reducing the chance of cross talk.
The impedance of the interface between the driver and receiver has also been defined. The load seen by the driver is specified to be 3kW to 7kW. For the original RS–232 standard, the cable between the driver and the receiver was also specified to be a maximum of 15 meters in length. This part of the standard was changed in revision “D” (EIA/TIA–232–D). Instead of specifying the maximum length of cable, a maximum capacitive load of 2500 pF was specified which is clearly a more adequate specification. The maximum cable length is determined by the capacitance per unit length of the cable which is provided in the cable specifications.
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