4
Basic Concepts
Communications
The Ronan X96S Level gage provides both 4-20 mA current loop and HART communications.
4-20 MA
For many years, the field communication standard for process automation equipment has been a 4-20 mA
current loop signal. The current varies in proportion to the process variable being represented. In typical
applications, a signal of 4mA will correspond to the lower limit (0%) of the calibrated range and 20mA will
correspond to the upper limit (100%) of the calibrated range. Thus, if the system is calibrated for 1 to 3
feet, then an analog current of 12mA (50% of range) will correspond to a level of 2 feet.
HART
HART
Field Communications Protocol extends the 4-20mA current loop standard to enhance
communication with smart field instruments. The HART protocol was designed specifically for use with
intelligent measurement and control instruments which traditionally communicate using 4-20mA analog
signals. HART preserves the 4-20mA signal and enables two-way digital communications to occur without
disturbing the integrity of the 4-20mA signal. Unlike other digital communication technologies, the HART
protocol maintains compatibility with existing 4-20mA systems, and in doing so, provides users with a
backward compatible solution. HART Communication Protocol is well established as the "de facto"
industry standard for digitally enhanced 4-20mA field communication.
The enhanced communications capability of intelligent field instruments employing the HART protocol,
offers significantly greater functionality and improved performance over traditional 4-20mA analog
devices. The HART protocol permits the process variable to continue to be transmitted by the 4-20mA
analog signal and additional information pertaining to other variable, parameters, device configuration,
calibration, and device diagnostics to be transmitted digitally at the same time. Thus, a wealth of additional
information related to plant operation is available to central control or monitoring systems through HART
communications.
Variables
There are two types of variables, communications variables and device variables.
Communication Variables
HART defines four device variables, PV (Primary Variable), SV (Secondary Variable), TV (Tertiary), and
QV (Quaternary). PV is assigned to the primary 4-20 ma loop . HART is also communicated over this loop.
SV is assigned to an optional secondary 4-20 ma loop.
