Can a Modicon PLC be a Modbus Master?

[Peter Nachtwey]

I have a customer that wants to communicate with our controller using Modbus. He is using a Modicon PLC, I have not been told which one. I am assuming it is a quantum. What instruction does one use to initiate Modbus transfers? I am hoping the MSTR block does the job.

 

[krk]

Hi Peter,

I take it you're referring to using the build-in Modbus port of the CPU rather than adding a serial comms card?

....if so then the answer is yes, you can configure it as a Modbus master to transmit to your (slave) controller.

MSTR is used for operations over the Modbus Plus network, the instruction you need is XMIT or XXMIT which will depend on which CPU and language you are using.

I think it was only supplied in the firmware on Momentums and is a purchased loadable for other CPU's.

Kevin H

**edit**
See this post over at control.com for more info on (X)XMIT

 

[SLC_Integrator]

krk is correct.

Quantum comes with MSTR blocks for MB+ and Mbus TCP comms.
If you want the Modicon to be a Modbus RTU master and poll from its serial port you need the XMIT instruction which doesnt come as standard but as a purchased loadable.

I found it a bit dumb that a $5k cpu doesnt have this function as standard yet a $500 Momentum does.

In my application I ended up making the other device the master (it was a Momentum) and the Quantum a slave. I really really didnt want to have to tell the client why his super-expensive Quantum wouldnt talk simple Modbus RTU.

 

[Peter Nachtwey]

Thanks guys

Now I know what to tell the customer and get him started in the right direction.

I found it a bit dumb that a $5k cpu doesnt have this function as standard yet a $500 Momentum does.

You and me both. I think it is odd that very cheap AD PLCs and ML1500s can be Modbus masters out of the box and Quntums can't.

I really really didnt want to have to tell the client why his super-expensive Quantum wouldnt talk simple Modbus RTU.

I don't make excuses for the screwed up PLCs. I just tell it like it is. Knowing all the facts saves time and money in the long run as long as there is an intelligent decision making process involved.

 

[bmyers]

Modbus

I am that customer. And I also find it strange that the expensive MODICON plc we just bought does not have a Modbus command, unless you pay extra. I am trying to get this to work with a Compact A984-145 that we have close to where we need the Motion Controller to be. But it appears that the XMIT function is only available for limited Compact CPU's and the -145 is not one of them.

I have been looking at the XMIT function, and to tell the truth, it looks kinda kludgy. I may end up with having to dedicate a Quantum or something else just for this app, one that I can talk to over MBPLUS, or go to an ethernet port model.

Still researching, and thanks for the help!!

Bobby Myers

 

[Peter Nachtwey]

I would get the Ethernet version. Once you get Ethernet going you will not want to go back. You can hang multiple devices on it. It is peer to peer. It is fast and most importantly it is better supported by Modicon with the MSTR blocks which are similar to the AB MSG blocks. They are easy to use. The difference in cost will saved by the time and effort required to get the system going and the future support.

Call Delta and ask for Jacob. Hopefully you already have an Ethernet card.