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if field commnictor 475 does not communicate with transmitter
what will you do ?
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Answer Posted / carl_ellis
1) Check for 24Vdc? Yes, but assuming the power supply is
24Vdc, don't expect the voltmeter to read 24v, unless you're
back in the control panel where the power supply is.
If you're out in the field where the transmitter is, you are
not close to the (-) side of the power supply, so where do
you put the black voltmeter probe? The only place for it
on the transmitter's (-) terminal, but measuring voltage
across the transmitter's (+) and (-) terminals only shows
what the transmitter's voltage drop is, not what the source
voltage is.
There is the voltage drop across all parts of the loop, the
transmitter, the load and the wiring. So checking voltage
across the transmitter's (+) and (-) terminals will never
show a full 24Vdc because of the voltage drops in the loop.
With short wiring runs and a 250 ohm load resistor, the
voltage across the (+) and (-) transmitter terminals might
be as high as 23V @ 4.0mA output, but could be as low as 10V
with a 20mA output across 500 ohms.
2) Checking to confirm that the transmitter is a smart
transmitter is a good idea. There are protocols other than
HART or Foundation Fieldbus (Honeywell's DE), and there are
lots of 'dumb' transmitters, too.
Some brands, like Siemens, allow HART to be enabled or
disabled, so the transmitter may be HART capable but have
the HART comm disabled.
3) Connecting a resistor in series is a good idea if the
load resistor is an unknown.
The article at the link below tells why a HART loop needs at
least 230 ohms of resistance
http://blog.lesman.com/2011/10/06/add250-ohm-resistors-to-make-hart-work/#more-68
The temporary use of a 250 ohm resistor is typical because
they're so common in a process plant.
4) Is the HART device at address 0 (zero)? The
communicator can talk to any address, but when connecting
point-to-point on home run wired transmitters, the
expectation is that the transmitter will be at HART address 0.
5) Experience will show that there are the inevitable
problems like
- broken or corroded communicator leads or clips that can go
'open' or be intermittent.
- power supply ripple or EMF induced noise on the loop
circuit can conflict with HART's 1200 baud FSk digital
communications
- some loop device, like a loop isolator or HART-unfriendly
loop indicator has stripped off the HART signal with a low
pass filter, leaving only the 4-20mA current signal
- wrong version DD so the communicator will connect but only
perform the so-called "common commands".
Tell 'em Carl sent you.
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