Serial Port Complete - Latest Microcontroller projects

Inside RS-232

gle +3V or +3.3V supply with signal voltages referenced to ground. Microcon-

trollers typically use positive logic, where the more positive logic level, or logic

high, is logic 1.

With TTL logic, a logic-low output must be no higher than 0.4V, and a

logic-low input must be no higher than 0.8V. A logic-high output must be at

least 2.4V, while a logic-high input must be at least 2V. Using these specifica-

tions, an interface can have 0.4V of noise without causing errors. These logic

levels are specified for the original, standard 7400 series of TTL logic and its

derivatives, including 74LS, 74F, and 74ALS TTL.

Most CMOS chips are specified for logic levels with wider noise margins com-

pared to TTL logic. A logic-low CMOS output is no higher than 0.1V, and a

logic-low input can be as high as 20% of the power supply, or 1V with a 5V

supply. A logic-high output is at least 4.9V, and a logic-high input must be at

least 70% of the power supply, or 3.5V with a 5V supply. Families that use

these logic levels include the 4000 series, 74HC, and 74AC logic.

Some chips have TTL-compatible inputs and CMOS-compatible outputs.

These chips can interface directly with either CMOS or TTL logic. Chips that

follow this convention include the 74HCT logic family, many microcontrol-

lers, and the 5V-logic interface on RS-232 and RS-485 driver/receiver chips.

3  

A convenient way to convert between 3V or 5V logic and RS-232 is to use one

of the many chips designed for this purpose. Maxim Integrated Products, Inc.

was the first to offer RS-232 interface chips that require only a +5V power sup-

ply. Since then, other companies have joined the market, including Intersil

Corporation, Linear Technology, National Semiconductor, Sipex Corporation,

Texas Instruments, and Zywyn Corporation.

Table 4-3: The voltage specifications for logic chips vary with the logic family and power-
supply voltage.

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Logic-low output (max.) (V) 0.4 0.1 0.1 0.1 0.1

Logic-high output (min.) (V) 2.4 4.9 2.9 3.2 3.5

Logic-low input (max.) (V) 0.8 1 0.6 1.0 0.8

Logic-high input (min.) (V) 2.0 3.5 2.1 2.3 2.0