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Old December 5th, 2017, 08:06 PM   #1
richleva
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Help!!! Modbus/hart troubleshooting

Hello,

My goal is to have a micrologix 1400 plc read data from a hart modem HRT-710. so I can read flow and totalizer and display them on HMI.

We have 3 other sites that are identical to this new site. I used all same programming for the PLC, flow meter (flowmag50W) and Hart modem (HRT-710).

the only difference is the new site has only one flow meter instead of 2 flowmeters, for that reason, I made sure to select point to point inside the hart modem (HRT -710).

I proved that the HART 710 modem is not defective by swapping with a unit from a different site.

I can read flow data and totalizer data from the flow meter via hart using the HRT-710 modem software, and using PC based software(Modbus master tool) but this was accomplished by using the RS 323 on the HRT 710. note that the HRT 710 has RS 232 and RS 485 to communicate, but only one can be used at the time.

When the PLC tries to retrieve data from the HRT 710 using a message instruction, it doesn't work. Message keeps saying error 37 " time out in local processor. this PLC programming is the same as other sites.

On the Modbus side, I also have one VFD that is communicating perfectly to the PLC, But not the hart modem?... I'm using 400 ohms resister for termination on both sides, I tried 250 ohms but didn't make a difference.

Also, the baud rate is same on all slaves and master.
And HART modem hart 710 is using net ID 3, PLC message instruction to pool the modem is using that same net ID 3.
and cable runs are very short.

I'm very confused, If any one has a suggestion. please let me know...!!
Thank you very much
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Old December 5th, 2017, 08:40 PM   #2
Ken Roach
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Interesting challenge !

My reading of your post is that you're using the Channel 0 port on the MicroLogix 1400 in Modbus RTU master mode, with the channel set for "485 - No Handshaking", to send Modbus RTU messages to both an AC drive and the HART modem.

Describe your actual RS-485 wiring from the MicroLogix to the AC drive and the HART modem.

Are you using two conductors, or three ? How are the signal common or comms grounds connected together ?

Have you tried to disconnect the AC drive and poll the HART modem only ?

This is a bit of a detour, but I'm curious about what you're doing with the drive and the MicroLogix. Are your start/stop/reference commands going over Modbus, or are you reading diagnostic data out of the drive ?
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Old December 5th, 2017, 10:43 PM   #3
danw
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There is no commonality between the HART protocol and the Modbus protocol, other than they can potentially use a common hardware bus at the master end.

The HART modem strips HART data off the field device, but that data is packed in HART comm packets, whatever those are and they're not Modbus packets.

HART enabled I/O is typically high end AI or AO cards for ControlLogix level controllers.

I think Moore might have a HART/Modbus gateway, and if it were a HART madter/Modbus server/slave then your 1400 would need to be a Modbus client/master.
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Old December 5th, 2017, 11:33 PM   #4
Mickey
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danw, I think the OP has this..

http://www.icpdas.com/root/product/s...y/hrt-710.html
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Old December 6th, 2017, 01:10 AM   #5
danw
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Thanks, Mickey, for the heads-up. It's a Modbus/HART Gateway, not a HART modem.

1.
Quote:
Originally Posted by richleva View Post
the only difference is the new site has only one flow meter instead of 2 flowmeters, for that reason, I made sure to select point to point inside the hart modem (HRT -710).
Most HART devices come with a factory default HART address zero for point-to-point HART comm, typically with a handheld communicator or a HART modem and software, like Pactware.

I would think that point-to-point mode on the HART side of the Gateway would mean the Gateway expects to talk to a single HART slave device at address zero. Addresses above zero are for multidrop or mux'd modes.

Is the field device at HART address zero?

2. Termination resistance for a point-to-piint or multidrop RS-485 network is typically 120 ohms, and only at ends of the network, typically master at one end and the last multidrop slave on the other end.

A HART modem needs a minimum of 230 ohms resistance in series in the analog 4-20mA loop to be able to develop sufficient signal amplitude to 'read' by the receiver. That resistance could conceivably be part of the HART side of the Gateway, maybe switchable in or out of the circuit, depending on how it's designed.
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Old December 6th, 2017, 01:32 AM   #6
danw
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1. Does the Parity setting on RS-485 side of Gateway match the PLC and drive parity settings?

2. Have you tried swapping the RS-485 A/B or (+)/(-) lines at the 485 side of hhe Gateway?

3. Is the any hardware or firmware 'switch' that flips betwen RS-232 and RS-485 on the Gateway?
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