Teledyne LXT-220 - Manual (Page 39)

Dissolved Oxygen Controller Operation
Teledyne Analytical Instruments 39
HART analog 2:, HART analog 3:, HART analog 4:
The next three displays assign the second, third, and fourth
variables. The assignments are changed in the same way as for the 4-20
outputs. See Section 3.3.2 4-20mA Output Assignments.
It is not possible to assign more than one HART digital output to
the same source. The current selection will be preceded by a < symbol
on the display if making it permanent would result in duplication.
HART analog: 5
4-20 output #4 is dedicated to HART. This output normally carries
the digital information superimposed on the 4-20 mA current. But if the
HART system uses the multi-drop mode then the current output of 4-20
#5 is disabled, with only the digital signal remaining. Several instruments
are connected in parallel in the multi-drop mode, so analog signaling is
not possible. The current that would have been output is still reported
digitally, as a percentage, and that will be sufficient for some applications.
But if an actual 4-20 current is required then one of the other 4-20 outputs
must be used to supply it, and this display is used to make the assignment.
The specified 4-20 must in turn be assigned to an appropriate source.
Note: If the HART analog output is assigned to an output other
than #5, then #5 cannot be assigned to anything. It must
be set to “missing” or “unused”.
The span settings for the selected 4-20 output correspond directly to
the HART lower and upper range limits, and they can be set either from
the keyboard or with command #35. If they are changed with the
command then the new value will be shown on the instrument's span
display for the selected output. If they are changed from the keyboard
then those values are reported digitally over the HART network.
If command 35 is to be used to set the limits then the HART 4-20
output must be selected first because the output span does not exist until
an assignment is made. The span settings must be readjusted or be
resent when the HART analog output assignment is changed.
The 4-20 supplying the HART analog output is normally assigned
to the same source as the primary variable, and the HART displays show
the engineering units for the upper and lower range limits as being the
same as those of the primary variable. It is possible to assign the analog
output to other sources, but other assignments are non-standard, and the
units displayed by the remote system will not be correct.