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MERCAP – INSTRUCTION MANUAL
7ML19981CM01.1
The MST9500 is equipped with three terminals, two of which are intended for connecting loop
power instrument cables. This connection is protected against incorrect polarity. The third
terminal allows the measurement of the current in the instrument cable with any digital
current meter instrument, without breaking the loop circuit.
The MST9500 also includes the following:
•
command push-button
•
command selection switch (4 positions)
position
1
record measured value for 4 mA
position
2
record measured value for 20 mA
position
3
field service use
position
4
TEST function
•
15 pin sub-D connector
field service use
The transmitter is powered by the current loop and needs at least 9-13 Volt (9 V at 22 mA, 13 V
at 3.6 mA) on the terminals. The maximum supply is 33 Volt. In case of higher voltages, the
safety diode will conduct, leading to an increase in power consumption. Some overload can
be tolerated indefinitely.
As a result of well-designed circuitry, the internal capacitance and inductance on the
terminals are isolated and do not interfere with safety calculations.
The MST9500 is equipped with the HART
TM
communication protocol so that settings and
information can be obtained and altered locally or remotely.
The internal diagnostic functions continuously monitor the correct operation of the
electronics. An error signal is generated if a failure or irregularity occurs.
MST9500 sends the signal current according to the NAMUR NE 43 recommendation. This
means that the current remains between 3.8 and 20.5 mA during normal operation. If the
process exceeds its normal limits, the current will be limited to 3.8 or 20.5 mA.
If there is a transmitter fault in the MST9500, or a test (position
4
) produces an error result,
the signal is changed to 3.6 or 22 mA.
Current values to signalize from digital transmitters
Measurement value (M)
Measurement value (M)
Fault- mA
Value (F)
Current values for signal detection
Fault- mA
Value (F)
F:=1
F:=0
F:=1
mA
mA
Fault- mA
Value (F)
Fault- mA
Value (F)