Transformer DOT conventions

Current going into the primary of the transformer and current leaving the secondary of the transformer are of the same polarity. This is shown on the transformer symbol as dot.

The upper circuits show Subtractive polarity and the lower circuits show Additive polarity. 







The ANSI symbols for Primary and Secondary are H1, H2 and X1,X2,X3 etc respectively.

If the terminal marked X1 is also on your left, it is subtractive polarity.  If the X1 terminal is on your right, it is additive polarity.  Additive polarity is common for small distribution transformers.

Ref:-
1. http://en.wikipedia.org/wiki/Polarity_(mutual_inductance)
2.  http://electrical-engineering-portal.com/understanding-transformer-polarity

HART communication meriam MFC4150

Hart communication with Meriam MFC 4150 communicator.
(a) Using near controller

The 250 ohm resistor is in series with the transmitter and power supply. HART communicator is connected parallel to the resistor.

(b) Using near the transmitter

Parallel resistor across the hart communicator can be omitted if the total cable length or series resistance > 250 ohms and less than 1100 ohms.
Bottomline is that there should be a voltage across a resistor which is > 1 volt and less than 5 v for the HART communicator to communicate with the transmitter.