Delta Controls 2HT - Manual (Page 9)

002522-2HT 180210.doc 9
Loop load
The required loop voltage is based on the loop resistance. To
determine the total resistance, add the resistance of all the units
within the loop (except the transmitter). The required supply
voltage can be determined by reference to the graph. Note that
there must be a minimum of 250 ohms within the loop for digital
communication to take place.
Factors to be considered.
The ideal system with no cross-talk, this assumes that the PSU has zero
impedance at the HART signalling frequencies. Note that separate positive and
ground lines are run to each instrument. Cables are spaced, or screened, to
prevent capacitive coupling between loops.
The HART protocol requires the PSU series resistance ( RS ) to be less than 10
ohms. Most power supplies will meet this requirement, but with the addition of
distribution wiring and in line fuses care must be taken. If required a 10000 uF
electrolytic capacitor of suitable working voltage may be connected across the
common terminals for the HART loop.
Common resistance (RC) this can be caused by high cable resistance, or by
grounding multiple loops in the field rather than in the control room. To prevent
high cable resistance resulting from grounding null loop, each loop wire should
be wired back to the psu.
COMMISSIONING
The Commissioning and Transmitter Function sections of this manual assumes a blind instrument or
one fitted with a DELTA local display Configurator. Blind instruments can be re-ranged and zeroed as
standard, using the one touch Zero and Span Buttons.
When the transmitter is shipped it has a fixed upper and lower range limit (URL, LRL) that is dictated by
the sensor used. The upper and lower range values can be set, (URV, LRV) to any value from LRL to
URL so long as the minimum span is not breached. The URV can be set lower than the LRV this will
reverse the transmitter’s analogue output, and give a 20 to 4 mA output.
The engineering units that are displayed and transmitted by the unit may be changed.
There are four ways to re-range the transmitter, each one makes changes to the setting of the URV and
LRV.
1 :-
With the zero and span buttons that are fitted on a blind instrument.
2 :- Enter a URV and LRV directly from the “DCAL” configurator (D-CAL).
3 :- Use the Zero and Span options from the local display.
4 :- Using a Universal Hand-held communicator (e.g. Rosemount 375)
+VE
0 V
PSU
RC
Tx 1 Tx 2
+VE
0 V
PSU
RL 1
RL 2
Tx 1 Tx 2
+VE
RS
0 V
10000 uF
PSU
RL 1
RL 2
Tx 1 Tx 2
RL 2
RL 1