Ronan X96S DENSITY GAGE - Manual (Page 6)

Density Gage Manual 012407 2
Basic Concepts
Communications
The Ronan X96S Density 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 SGU, then an analog current of 12mA (50% of range) will correspond to a density of 2 SGU.
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.
Communications Variables
HART defines four device variables, PV (Primary Variable), SV (Secondary Variable), TV, and QV. 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.
Device Variables
The Ronan X96S Density gage has 5 device variables:
Device Variable Value
D1 Density
D2 Density in alternate units
Proc Temp Process Temperature
Pressure Process Pressure
Head Temp Head Temperature