Rockwell Automation T80004 Application Note Field Loop Configuration - Manual (Page 50)

Trusted
TM
AN-T80004 Field Loop Configuration
Issue 10 Jun 13 AN-T80004 50
potentials applied serially across components. Where lightning strikes are common, field wiring should
be protected by individual transient voltage surge suppressors which will reduce (but not eliminate) the
risk of damage from induced noise. These will still not protect against direct strike, so the site should
also have lightning rods fitted to deflect strikes away from plant and field devices.
HART interfacing
None of the Trusted analogue I/O modules will communicate HART protocol. Separate HART
multiplexers should be fitted to the loop to communicate with HART devices. These will not affect the
current loop operation.
HART is designed for adding digital communications to a simple 4-20mA current loop. A frequency-
shift keyed signal is superimposed on the current loop signal at frequencies far higher than the loop’s
main signal spectrum as shown. These frequencies can be easily filtered, and do not affect the Trusted
modules’ current loop control or measurement.
The HART multiplexer should be connected across a loop impedance, so that it is able to drive a signal
round the loop. The power supply may be considered as part of the loop, since it appears as a short
circuit to the signal.
Some possible connection methods are shown below. The HART multiplexer is shown as a grey box.
These permutations are also suitable for isolated analogue inputs; the essential point is that the HART
transmitters should see an impedance across their terminals.
24V
Channel
0V
250R
24V
Channel
0V
250R