Hi you all!

I have a question about Modbus Masters.

I want to know how i can solve the following situation:

I have one system that communicates, using Modbus protocol and RS485 bus, with some network analyzers. I would like to put another master on the line, besides the mentioned above, to make some requests to this network analyzers to.

The problem is that probably with two masters on the line the messages will get screwed and there will be collision for sure.

Is there any hardware device (to which i connect both masters) that i can use to connect both systems that is able to manage the requests on the line? This way this device would buffer the requests from the two masters and avoid problems on the line.

Or is there other way to solve this problem?

Thank you all

LPS

 

The MARC Multii-Port:

http://www.miille.com/din366-10p.pdf

is commonly used for connecting multiple Modbus Masters to a network of one or more Modbus Slaves. It works as you describe by buffering the messages and only replying to the master that made the request. You can contact me directly if you have any questions. Jmiille [at] miille.com

Thanks,
Miille Applied Research Co., Inc
http://www.miille.com/
Jerry Miille

 

Hi,

I think that you would have to find a way to sync the two masters somehow. When one of the masters is communicating, then the other one doesn't, What type of masters are you using, PLCs or PCs?
Also are you using ASCII or RTU?

If you are using the RTU framing protocol, you could extend the silent interval of one of the masters. Instead of a silent interval of 3.5 characters, try setting one of the masters silent interval to something like 5 characters. This might give you enough of a window to perform the communications.

I never had to use two masters before, so if you try it, please use caution.

criders

 

Hi There,

HMS makes a gateway that is Modbus RTU slave to Modbus RTU slave. Connect one master to one of the slaves and the other master to the other slave. The gateway has a fixed mapping of 512 Bytes In and 512 Bytes Out.

For more info, go to:

http://www.anybus.com/support/support.asp?PID=234&ProdType=Anybus X-gateway

or send me an email at:
support [at] hms.se

//Patrick L

 

Check out the PLC09 Arbitrator (www.calta.com). It was developed for exactly this application.

Regards
Lorne

 

In the modbus network, there should no more than one master running at the same time.

 

It is possible to use master multiplexer.
For example: http://www.marcomweb.it/InfoProdotti/157.aspx

With this equipment the two master can have different speed.

Best Regards

 

You are absolutely correct, only one master "at the same time". It is possible to have multiple masters communicating with the same slave if you have some sort of "gatekeeper" module that will control access to the slaves. That is the purpose of the Multii-Port module that I recommended earlier in this thread.

Jerry Miille

 

We manufacture a Gateway that allows two Masters over the same Modbus Slave devices. It is our DX Series and you can find technical information on our Internet site at www.asconcorp.com/library/pdf/delta2/DX-DY_Brochure.pdf

I hope this helps with your application.

 

Hey Jerry, So, hypothetically, in a SCADA application that I have, I am only sampling every 5 minutes across 20 modbus devices. SO, if I were to stagger the two masters such that they scan 2.5 minutes apart, would I still need a gateway?

thanks,
James Lane

 

As long as you can synchronize your masters there will be no problems. Since I don't know anything about your Modbus Masters I can't offer any suggestions on how you would do the synchronization.

Jerry Miille
Miille Applied Research Co., Inc
http://www.miille.com/
Jerry Miille