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Micrologix 1100 to Agilent 34980A
Need to connect Micrologix 1100 PLC to Agilent 34980A.

I am new to all this stuff and any help is much appreciated.

I have several Micromotion transmitters that output Modbus protocol. The transmitters are capable of outputting both mass and volume flow, both of which I want to be able to record on the Agilent. Currently we are outputting a 4-20mA signal from the agalog output on the transmitter which is going to the Agilent. How much is it going to take to get the Modbus output to the Agilent. The people at Agilent have said that the system cannot read modbus protocol, and the modules we are using now only take analog inputs, so I am kinda stuck here. I have a Micrologix 1100 PLC at my disposal which I have been told works great at doing what I want done, but I haven't been able to see where it would work.

Thanks in advance for any help on the subject.

The Agilent rack is a data acquisition rack. What is it hooked up to though? A PC? If so, then why can't you just connect the Micromotion transmitters directly into the PC using Modbus?

I don't see why the Agilent rack would need to have anything to do with Modbus. The Micromotion singals are already converted into something a PC can read directly. The real question will be about what your application software needs to do to read from this new signal source.

I think I see what you are saying. We are using Visual Basic to compile the data from the Agilent rack, I'm assuming there is a way to program this in so we can capture the modbus information in with the rest of analog signals from the agilent? Again little to no experience with DAS so any help is very appreciated.

You will need a Modbus driver that will work with MS Visual Basic, and you will need someone who can modify the MS VB program.

You will also need a way to plug the Micromotion transmitters into the PC. If the connection is RS-232, you will need RS-232 ports. If the connection is RS-485, you will need either an RS-485 card, or an RS-485 to Ethernet (or to RS-232) converter. If the connection is Ethernet, you will need an Ethernet port and an Ethernet switch.

At the moment your DAQ program is reading the Agilent rack (presumably through some sort of driver). The program will need to be modified to also read the Micromotion transmitters via the Modbus driver.

You will need to talk to someone who can change the DAQ program for you. That person should also be capable of picking out the correct components based on the details of your installation.

I am currently retired from an Emerson Rep and I have over 25 years experience with Micromotion and Modbus and still provide VB and C++ services to my former employer. All Micromotion Transmitters support Modbus over RS485 communications. I would be willing to help you with this. To start with I would recommend a B&B 485SD9TB RS232 to RS485 for the Win PC Application because it does not require any special KeyUp KeyDown handling from the VB Application.

I have been instructed not to go with the option of using the VB modification. We already have the AB MicroLogix 1100 PLC so we want to try to use that to somehow send the modbus signal to the Agilent rack. I still don't know if there is a way to convert the Modbus signal to two seperate analog signals using the PLC. Is this at all possible?

By Jerry Miille on 13 November, 2009 - 7:59 pm

I am not sure I follow your original post but if it requires using a MicroLogix 1100 PLC to generate Analog Outputs of any kind, then, as far as I know, you are out of luck. There may be expansion modules available that I am not aware of but the MicroLogix 1100 does not have analog outputs as far as I know.

Now, there a lots of ways to get Modbus serial data converted to a form that could be sent to an Integer file in the MicroLogix for subsequent action, but, as far as I know, there is not an Analog Output function available.

Jerry Miille

The PLC would serve as a Modbus Host over RS485 to all MM Transmitters. The PLC program would then take the flow rates and scale them for output as an Analog 4-20mA. You need a PLC programmer. But before you go there, is it going to be possible to daisy chain a shielded twisted pair of wires to all the MM Transmitters from the PLC?

Are you sure you even need Modbus? Most all MM Transmitters support 2 x 4-20 mA Analog Outputs that can be configured to any of the measured variables. What MM transmitter model numbers do you have and what variables do you need ?

We have the base models of the MM. off the top of my head I think it's the 1700 so it only has one analog output. There is talk of just buying the prolink software and using that to change the mass flow to volume flow, but it is more than we'd like to pay. We already have the micrologix so we'd like to use that but if it's going to be more efficient to just buy the software. we may have to bite the bullet. I know nothing about programming so it's looking like the software might be the better option. any thoughts? is there any other way to change the output than by purchasing the software?

You can configure the MM Transmitters with almost any generic Modbus Host. For example, the register that assigns the process variable to the Primary Variable 4-20mA is documented as Modbus Register 40012

Your existing Micrologix 1100 can easily read modbus registers into its own internal data registers. Then your PC data acquisition software should be able to read the Micrologix data registers through the ethernet port.

No need to 'go through' the Agilent.

Check out Automated Solutions' website for a Modbus (master or slave) ActiveX Control that can easily be added to Visual Basic applications.

Free, fully-functioning, 30-day trial version is available for download using this link:


By Wassim Daoud on 30 October, 2009 - 11:13 am

This is a common challenge in the control systems world usually resolved by using OPC. To learn about OPC check out this link here:

What is the size of your project?

Wassim Daoud
Golbal Solutions Architect

I am not sure if the Micrologix can do ASCII serial, but if it can then you should be able to use this converter:

Am I missing something? What does the IEEE 488.1 and 488.2 standard have to do with RS485? The MM Transmitters use Modbus over RS485 (or Hart over a 4-20 mA loop).

By Harold Ennulat on 20 July, 2010 - 10:22 pm

Did you ever get the Micromotion flowmeters talking Modbus to the AB MicroLogix 1100 PLC via the RS485 com port(s)?

I have to do the same thing with a MicroLogix 1400. The capability is there, but to build all that PLC messaging and encoding/decoding logic looks like more work then the project can allow. I'm looking for some PLC sample code that does this.

I can be reached by email at if/as preferred.


FieldServer ( has a Modbus RTU to EtherNet/IP gateway that is Encompass approved.


By Jerry Miille on 21 July, 2010 - 10:44 pm

I am going to jump in here and propose another solution. Instead of doing all the work in the PLC, why not use an external protocol converter to do all the work for you. Yes, this adds some dollars to the cost and some additional hardware but there is NO PLC PROGRAMMING needed. Our Omnii-Comm product can take care of getting your flowmeters connected to the PLC. We have free example configuration files for the Omnii-Comm freely available from our web site.

Please click on this datasheet for additional details and here if you have any questions.


Jerry Miille

Miille Applied Research Co., Inc.

By Bob Peterson on 22 July, 2010 - 8:19 am

I am going to jump in here and propose another solution. Instead of doing all the work in the PLC, why not use an external protocol converter to do all the work for you. Yes, this adds some dollars to the cost and some additional hardware but there is NO PLC PROGRAMMING needed. Our Omnii-Comm product can take care of getting your flowmeters connected to the PLC. We have free example configuration files for the Omnii-Comm freely available from our web site.

I like using these kind of things, but I have also been leary of doing so due to the potential for the supplier to go out of business and leaving me with an unsupportable configuration. This has happened to me so many times with so many different automation products that I am very gun shy these days about anything not from a solid, long term company. And even that is no real guarantee, but it reduces the risk substantially.

OTOH, I am 52 now and I only have to worry about stuff like this for another decade or so.

By Jerry Miille on 25 July, 2010 - 9:14 pm

I understand your concerns Bob, but I will have to point out that we have been in business for 30 years and this product has been in service for the last 17 years or so. It is NOT A NEW product, but a well-tested and seasoned product.

But, you are correct, companies do "go out of business" or are acquired and the product lines can "die" leaving you stranded.

On that line of thinking you have to take your chances but I would like to point out that a lot of bigger companies that were there when we started (1980) are no longer around. It is not unusual for a major automation supplier to "change horses" and leave you stranded. Could be that they had a new product that was better or they were acquired by someone else or they just closed the doors. Doesn't matter, what you need is a product that works, has a history of support and an expectation of being there in the future. Have a look

The ML1100 can receive Modbus integers. you can buy an axillary module (I use AB 1762IF2OF2, 4 channel, 2 analog In, 2 analog out, configurable to 0-10VDC or 4-20mA) and program scaling functions for the Modbus integer input, then output 4-20mA... Very easy logic.