3-21
On-line Operations
5. Record the actual value from the reference meter, and enter it at
the “Enter Meter Value” prompt. The communicator prompts you
to verify whether or not the output value equals the value on the
reference meter.
6. Select
1 Yes
if the reference meter value equals the transmitter
output value, or
2 No
if it does not.
If
you select
1 Yes
,
then
proceed to Step 7.
If
you select
2 No
,
then
repeat Step 5.
7. Select
OK
at the “Setting fld dev output to 20 mA” prompt, and
repeat Steps 5 and 6 until the reference meter value equals the
transmitter output value.
8. Select
OK
after you return the control loop to automatic control.
Scaled Output Trim
The
Scaled D/A Trim
command matches the 4 and 20 mA points to a
user-selectable reference scale other than 4 and 20 mA (2–10 volts, for
example). To perform a scaled D/A trim, connect an accurate reference
meter to the transmitter and trim the output signal to scale as outlined
in the Output Trim procedure.
Apply Values
The
Apply Values
command uses process inputs to set the 4 and 20 mA
values. Calibrating to applied values is a method of adjusting the
analog output to a plant-standard input device. Use a decade box, an
RTD or thermocouple simulator, or a sensor in a known temperature
bath as an input. This function is similar to calibrating an analog
transmitter using more lengthy traditional calibration methods
Multidrop Communication
“Multidropping” refers to the connection of several transmitters to a
single communications transmission line. Communication between the
host and the transmitters takes place digitally with the analog output
of the transmitters deactivated. Many of the Rosemount SMART
FAMILY transmitters can be multidropped. With the HART
communications protocol, up to 15 transmitters can be connected on a
single twisted pair of wires or over leased phone lines. Multidrop
installations are not recommended where intrinsic safety is a
requirement.
The application of a multidrop installation requires consideration of the
update rate necessary from each transmitter, the combination of
transmitter models, and the length of the transmission line.
Communication with the transmitters can be accomplished with
commercially available Bell 202 modems and a host implementing the
HART Protocol. Each transmitter is identified by a unique address
(1–15) and responds to the commands defined in the HART protocol.
HART Fast Keys
1,2,3,5
HART Fast Keys
1,2,3,1
