Agilent Technologies 54621A Programmer's Manual page 42

Programming over RS-232-C
This section describes the interface functions and some general concepts of the
RS-232-C interface. The RS-232-C interface on this instrument is
Hewlett-Packard's implementation of EIA Recommended Standard RS-232-C,
Interface Between Data Terminal Equipment and Data Communications
Equipment Employing Serial Binary Data Interchange. With this interface, data
is sent one bit at a time and characters are not synchronized with preceding or
subsequent data characters. Each character is sent as a complete entity without
relationship to other events.
IEEE 488.2 Operates with IEEE 488.1 or RS-232-C
IEEE 488.2 is designed to work with IEEE 488.1 as the physical interface. When
RS-232-C is used as the physical interface, as much of IEEE 488.2 is retained as the
hardware differences will allow. No IEEE 488.1 messages such as DCL, GET, and END
are available.
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