HART COMMUNICATOR

R

Thread Starter

RR

Hi Group,

I just came across something new with the HART communication on field instruments. We are equipped with Bailey Infi 90 dcs with almost 8 nodes. (Say PCU 1 TO 8) Recently the transmitters connected to PCU6 stopped communicating with HART when connected across TRANSMITTER'S POSITIVE and NEGATIVE terminals. Checked for the loop resistance, grounding, restarted the power supply module etc and found normal.

We continued trouble shooting with reference to loops from other PCUs.

The result is:
HART comm is success when connected to transmitter terminals from (Both on transmitter and cabinet side) for all PCUs except PCU6.

1. Positive to Negative
2. Positive to Ground
3. Negative to Ground.

- For pcu6 only Negative to Ground is able to communicate.
- My so far (mis..?!) understanding on HART signal is that it will only communicate from Positive to Negative terminals.

- This scenario is strange to me.

- Suspecting my DCS loop i tried the same testing on another stand alone ABB 800XA system in my plant and experienced the same. i.e, good communication for,

1. Positive to Negative
2. Positive to Ground
3. Negative to Ground.

My questions are;
1. Is it common that HART can communicate on above three type connections? If so how/ why?

2. What may be the reason that it couldn't communicate for PCU6 ,except Negative to Ground?

Please help.

Notes:
All that i tested were DCS loop powered transmitters. No issue with 4 wire transmitters.

Thanks,
RR..
 
> 2. Positive to Ground
> 3. Negative to Ground.

1) "Ground" means?
- power supply negative?
- a bonded earth ground in the panel?
- the steel pipe driven into the ground that the transmitter is mounted on?
- the receiver (analog input) negative?
- something else?

2) Is either side of the power supply earth grounded?

3) Does any of the loops have an intrinsic safety barrier in the circuit?

4) Is there any other component in any of the loops, like a loop isolator or loop powered indicator, whatever?

5) Are all of the loop powered devices in question (those that communicate and the one that doesn't) on the same DCS power supply?

6) Are there any components, like capacitors or resistors connected to the analog input, other than the IR dropping resistor?
 
Hi David,
Thanks for your interest shown.


> 2. Positive to Ground
> 3. Negative to Ground.

1) "Ground" means?
-- a bonded earth ground in the panel?
- 2) Is either side of the power supply earth grounded?

Yes.Both ends are grounded.
3) Does any of the loops have an intrinsic safety barrier in the circuit?
No such barriers in the loop.

4) Is there any other component in any of the loops, like a loop isolator or loop powered indicator, whatever?
Most of the loops have Acromag card connected to it.The cable is terminated on <pre>
NTAI05 . TX
TB1-1------------------------------- +
TB1-2 -------:---------------------- -
:
: ACROMAG CARD
.................. 1
....... 2
:
___
_
. (ground) </pre>
(NTAI01-5 TB1-2 to Acromag card terminal 1
Acromag card terminal 2 to Panel Ground Bus bar.)

We use to hookup hart on acromag terminals 1 and 2 usually to access the transmitter for simulation and other maintenance purposes.
5) Are all of the loop powered devices in question (those that communicate and the one that doesn't) on the same DCS power supply?
No. Those that can communicate are all on different power supply.

6) Are there any components, like capacitors or resistors connected to the analog input, other than the IR dropping resistor?
I can say only the Acromag card.

Hope this helps.
Thanks.
RR
 
1) >Those that can communicate are all on different power supply.

A power supply acts as a low pass filter, which strips off the (relatively high frequency compared to the DC current) 1200 baud HART signal.

The reason for the HART Foundation's minimum requirement of 230 ohm loop resistance is to provide sufficient resistance for the HART 1200 baud signal to develop a voltage of sufficient magnitude to be detected by the HART receiver.

I suspect that differences in power supplies between the functioning loop and the dysfunctional loop are somehow affecting the loop to where the HART signal is not readable, but the DC current is. Increasing the loop resistance might help.

2) I'm not sure what component is which in your graphic. Is the Acromag the field transmitter or an analog input? (Acromag makes both)

The one Acromag current input unit that I have a manual for, the 9678EN, says it uses 200 ohms input resistance for its current input channels.

200 ohms is low; HART Foundation says a minimum of 230 ohms is needed.

3) If I understand your graphic correctly, it appears that you have the possibility of a developing a ground loop, having earth grounded both ends of the loop. There's a possibility that high frequency noise is disrupting the HART signal, but the noise isn't 'seen' by the analog input, which typically use some form of low pass filtering for current signals.
 
Top