Modbus serial tree topology??

Hi,

Is there a way to make a branch in MODBUS serial?

Thanks in advance.

Vicente

im not sure what you are asking. do you mean have more than 1 modbus device hooked up to your modbus master?

I believe this is in the standard for Modbus/TCP but not sure about serial. It will really depend on what your hardware supports.

Can you post more about your hardware and perhaps a diagram of the topology you are trying to achieve with your network?

A few years ago I was involved in a project that had a Modbus master with three slaves. The drawings for the system depicted the Modbus serial cable as a trunk line, but the contractor who did the installation was more accustomed to running cat5 ethernet cable so he installed the MOdbus cable in a star topology just as he would have installed an ethernet network.

To make a long story short, it didn't work. The contractor had to redo the network cabling in a trunk configuration.

That's an extreme case of branching. Somewhere between a long trunk with branches of zero length and a trunk of zero length with long branches lies the point at which the cabling will fail. I can't predict where that point lies, but the longer you make the branches, the greater the risk of failure.

Maybe there is a rule of thumb that limits the total length of all branches to less than x.yy percent of the total cable length????

Serial is a nickname for a number of physical layer types. Among them are RS-232, RS-422 and RS-485. 232 is point to point only. 422 and 485 are bus topologies. If your topology is a bus type then yes, you can make a branch.

Thanks to all. I attached the topology, this is that i need to make. Is possible?.
If yes, Which device, active or pasive, i need to put in the red circle? Nothing, a "T" or a repeater.

regards.

I would put a modbusTCP to modbus converter at the start of each branch. Lots available, I use the ones from Schneider, just make sure the modbus side is RS485 and your modbus master is able to be ModbusTCP

Usually it would be set up like this:



You are using RS485 right?

ndzied1 said:

Usually it would be set up like this:

You are using RS485 right?

Click to expand...

Yes, I'm using RS485, and the master is a MCM Prosoft module in a ControlLogix. (i have the two port occupied)
Ndzied1, I can't see the picture that you attached. Can you try to upload again?..thank you.

See links below.

http://www.bb-elec.com/technical_library.asp#RS422

http://www.bb-elec.com/tech_articles/485_star_network_illustration.asp

it may or may not work right. its anyones guess. either run the wire correctly so that it will definitely work or buy a rs485 splitter of some sort. I am sure black box makes something that you could use.

Since it is rs485, then you should be able to have the master in the middle of a daisy chained network. Just move your termination resistors/jumpers to the nodes at the physical ends of the network.

OkiePC said:

Since it is rs485, then you should be able to have the master in the middle of a daisy chained network. Just move your termination resistors/jumpers to the nodes at the physical ends of the network.

Click to expand...

hmmm, so you are saying put the master anywhere on the line, but instead of terminating at the master terminate on both ends? never heard of that. I am interested in seeing if that would work.

Mickey said:

See links below.

http://www.bb-elec.com/technical_library.asp#RS422

http://www.bb-elec.com/tech_articles/485_star_network_illustration.asp

Click to expand...

OkiePC said:

Since it is rs485, then you should be able to have the master in the middle of a daisy chained network. Just move your termination resistors/jumpers to the nodes at the physical ends of the network.

Click to expand...

Mickey and OkiePC, I thing that you gave me the correct answer.
With the answer of Mickey I confirm the branch topology in a datasheet of a RS485 repeater, and looking this diagram topology I thought in the same thing that explain OkiePC, you can put the Master anywhere in the bus.
OkiePC I'm sure that you have the best answer, we'll try it and i'll bring the result to confirm you.

Thanks to all.

Sorry

I accidentally deleted the image. Here is what I meant to post (just got back to a computer I could post from).