Delta Controls 2HT - Manual (Page 11)

002522-2HT 180210.doc 11
MULTI-DROP CONFIGURATION
Multi-dropping transmitters refers to the practice of connecting several units to a single transmission
line. The analogue signal is switched to 4 mA on all units and communication takes place digitally only.
Each message is sent with a unique number that identifies the field device.
The HART® system limits the number of instruments that can be connected in this way to 15. For
Intrinsically Safe installations, maximum permissible circuit current reduces the limit to four. When a
multi-drop system is designed, consideration must be given to the length of transmission line, update
rate required from each transmitter and total power requirement. Multi-drop systems are not
recommended for Intrinsically Safe installations. Each transmitter is identified by a unique number from
1 to 15 and responds to HART® commands that are sent to that address.
Note that the transmitter is sent from the factory with its address set to 0 this disables multi-drop and
allows 4 to 20 mA and digital communication only.
There is no way to change the polling locally. This is for safety reasons, so that the address can only
be changed once digital communication has been established from Hart master.
The use of a separate HART® Multiplexer is permissible, but outside the scope of this manual.
TROUBLE SHOOTING
The integrity of a HART® loop can be validated using several methods, if a local display is fitted the
{SHOWHART}, {FIXLOOP} and {SELFTEST} methods provide a high level of confidence, for lower level
testing or if a local display is not present the use of a simple oscilloscope and voltmeter can be used.
Note: in an Intrinsically Safe application restrictions may apply to the instruments being used.
Analogue Loop Tests.
Is the voltage at the instrument, across the terminals, above the lift-off voltage of the instrument,
typically 9 to 12 volts, see the functional specifications?
Measure the DC voltage across the load resistor, derive the loop current using the calculation :-
Loop Current = DC Voltage / Load Resistor
This should be between 4 and 20 mA and representative of the measured prime variable, unless the
device is in multi-drop mode where the current will approximately equal 4 mA multiplied by the number
of devices within the loop.
Is the loop grounded properly, normally in one place, at the power supply?
Is the screen of the cable grounded at one end only, normally at the control room?
Are the cables isolated from other high current cables?
Is the cable of the correct grade and not to long, see cable selection and signal level?
Are the barriers HART compliant and is the power supply low impedance?