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Maxim > Design Support > Technical Documents > Application Notes > Interface Circuits > APP 882
Keywords: RS-232, rs232, TIA/EIA-232, MegaBaud, RS-232 compatible, RS-232 compliant, SIO, serial
data, auto shutdown, autoshutdown plus, auto wakeup
APPLICATION NOTE 882
RS-232 Features Explained
Jan 01, 2001
Abstract: RS-232 features are explained in a way that makes it easy to select an appropriate RS-232
communication IC to meet any system need. Special attention is given to, signal swings, slew rates,
enhanced ESD protection as well as data rates up to 1Mbaud. The concept of RS-232 compatible vs.
RS-232 compliant is introduced where system needs may suggest high data rates or low supply
voltages. Products are identified to meet low-power system needs with automatic shutdown to low-power
mode and auto wakeup, low supply voltages, high ESD protection, programmable logic thresholds, and
high data and slew rates.
As of January 2001, Maxim makes more than 124 RS-232 parts. To help you choose the most
appropriate part for an application, this article explains the different features available.
Power Supplies
A separate article, Selecting and Using RS-232 Interface Parts For Your Power Supply Voltages,
describes the RS-232 parts available for different power supply voltages.
Data Rate
Just about all Maxim RS-232 parts will transmit and receive data at up to 120kbps, with most parts being
capable of rates as high as 250kbps. All of this is done while conforming to the 30V/µs maximum slew
rate designated in the RS-232 specification (officially called TIA/EIA-232-F). Higher data rates are
available, but require special mention.
MegaBaud
MegaBaud is the term Maxim uses for RS-232-logic-level-compatible data rates that are 1Mbps or
higher. A keen observer would notice that the word "compatible" is used here as opposed to "compliant."
This is a subtle and important point that needs further explanation. Buried deep within the RS-232
specification is a limit that states that the slew rate of a transmitter must be less than 30V/µs. This limit
was specified in the interest of making RS-232 a simple physical interface. Without this limit, much
closer attention must be paid to things such as radiated emissions and some transmission-line effects.
Although this slew-rate limit makes the physical interface simpler, it also effectively limits the maximum
data rate that can reasonably be used. Maxim makes a series of parts that are capable of MegaBaud
operation and meet the RS-232 specification except for this slew-rate limit. To ensure that the physical
interface remains simple, these series of parts are still slew-rate-limited but have a higher limit than
allowed in the RS-232 specification (150V/µs in the case of the MAX3237E).
What does all of this mean? For starters, these parts are not strictly RS-232-compliant. In other words, if
a MegaBaud-capable part is plugged into an ordinary RS-232-compliant port, proper operation cannot
be guaranteed even at data rates as low as 20kbps. In reality, however, the lower data rates will
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