Advantech BB-USR604, User Manual

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BB-USR604
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The original Modbus specification included two possible transmission modes: ASCII and RTU. Modbus RTU mode is
the most common implementation, using binary coding and CRC error-checking. Modbus ASCII messages (though
somewhat more readable because they use ASCII characters) is less efficient and uses less effective LRC error
checking. ASCII mode uses ASCII characters to begin and end messages whereas RTU uses time gaps (3.5 character
times) of silence for framing. The two modes are incompatible so a device configured for ASCII mode cannot
communicate with one using RTU.
Although some newer equipment using Modbus incorporates USB interfaces, most legacy equipment implements
RS-232, RS-422 and RS-485. Most new PCs have eliminated RS-232 ports as standard features and RS-422/485 was
never a standard feature. As a result USB to serial converters such as the BB-USR604 are often necessary to
communicate between PCs and legacy systems that implement Modbus protocol.