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Tuesday, October 16, 2018
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Conversion of MODBUS Slave Output to Analogue 4-20mA
I want to convert a Modbus output on a flow meter to 4-20mA analogue signal.

I need a 4-20mA output from a RMG type TEC24 gas flow meter (Modbus) I installed. I bought a DAT3024 converter from DATEXEL.

Kindly assist me as I don't seem to get the necessary configuration.

You need to use PLC or HMI to receive data from TEC24 as shown in link below at page 56.

https://www.rmg.com/wp-content/uploads/2017/05/ec24_manual_en_01.pdf

Then scale data from 4000 to 20000 (4 -20 mA) at register 40015 to 40018 of DAT3024 as shown at page 3.

https://www.datexel.com/datexel-user-guides/dat3024-voltage-output-from-rs485-modbus.pdf

This link may be more helpful regarding the DAT3024 that a previous poster mentioned.

https://www.datexel.com/rs485-4-20ma-analogue-output-module-dat3024.html

Other than this, you need to be more specific as to what you are having problems with.

William (Bill) L. Mostia, Jr. PE
ISA Life Fellow,
Winner of the 2018 ISA Raymond D. Molloy Award

"No trees were killed to send this message, but a large number of electrons were terribly inconvenienced." Neil deGrasse Tyson

Any information is provided on a Caveat Emptor basis.

Hello All,

Thanks for the replies. But we have a limited budget to actualize this installation and using a PLC as a Modbus master + a HMI is not possible for us due to the cost.

We already have a flowmeter which is a Modbus Slave. What we simply want is a 4-20mA output denoting the flow rate. Is there no other protocol converter we can use to achieve this than passing this fieldbus output through a PLC or control system?

NOTE:The flow meter is being used in a zone 1 environment and there is possibility of explosion.

By W.L. Mostia on 2 October, 2018 - 10:30 pm

Are you sure your flowmeter doesn't have a 4-20 ma output? If not, you maybe able buy an I/O card to add a 4-20 mA from your flowmeter. The RMG TEC 24 documentation seems to indicate that if you have a meter powered by the mains (e.g. not battery powered), you might have a built in 4-20mA output (see page 2 of below documentation). I would talk to the manufacturer and see if they can help you with what you are trying to do.

https://www.rmg.com/wp-content/uploads/2017/05/tec24_leaflet_en_02.pdf

Your problem is that you need a Modbus Master to talk to a Modbus slave (your flowmeter). You might talk to Automation Direct (https://www.automationdirect.com/adc/Home/Home) about a small PLC or HMI that talks Modbus and can output 4-20 mA or some other device. They tend to have inexpensive solutions to automation applications.

The DAT 3024, I believe is a MODBUS slave device, and you also need a DAT9000 as a Master to completer the MODBUS circuit. See the below applications note.

https://www.datexel.com/how-to-obtain-a-4-20ma-out-of-a-multivariable-modbus-flowmeter.html


William (Bill) L. Mostia, Jr. PE
ISA Life Fellow,
Winner of the 2018 ISA Raymond D. Molloy Award
Sr. Functional Safety Consultant
SIS-SILverstone, LLC.
http://www.sissilverstone.com
281-728-3722

"No trees were killed to send this message, but a large number of electrons were terribly inconvenienced." Neil deGrasse Tyson
Any information is provided on a Caveat Emptor basis.