Modbus TCP 2

[xJAyx]

I am working on a SCADA system upgrade and the owner wants to use Modbus protocol on an ethernet radio network. I am having trouble finding any useful information on Modbus TCP protocol. Does anyone know of any good websites/documents which could help me out? I'm mostly looking for a list of features, specifically whether it supports multiple masters, peer to peer operations (submasters), time stamped events or unsolicited messaging.
I know modbus RTU does not support a lot of these things, but I am looking specifically at modbus TCP.

Thanks for your help!

 

[ZapSib]

I would suggest to visit

http://www.modbus.org/

zapsib

 

[danw2]

Modbus RTU and Modbus ASCII do not support multiple masters. Both are based on the open Modbus protocol

Modbus plus does support multiple masters but is limited to those vendors who license the technology.

Modicon and Schneider supports Modbus Plus.

http://www.modicon.com/opennetworking/modbus/

 

[Noway2]

I may be wrong here, but the impression I received of Modbus TCP from the one installation I did some work at that used it, is that it is Modbus (binary or ascii - usually binary) is transmitted over an ethernet link as opposed to an rs-485.

I base my impressions on the fact that the commands that were transmitted and received were standard Modbus RTU and they used a combination of ethernet switches and ethernet - rs485 converters for the devices that only supported rs485.

 

[xJAyx]

Actually there are slight differences between Modbus TCP and Modbus RTU. They are quite similar though, and there is such a thing as Modbus RTU on TCP, which is different from Modbus TCP and which is possibly what you were working with.

 

[LAsludge]

Modbus TCP is modbus running over ethernet. The support of the specific features will depent on who's Modbus TCP driver you use.

Try Software Toolbox's TopServer or Wonderware's Modbus ethernet

 

[knji]

The ability to have multiple masters on a Modbus/TCP network depends on how the masters are implemented. If a master simply means that the device can initiate a conversation (note that Modbus is half duplex by nature), then I do not see any reason why you cannot have multiple devices polling each other over the full duplex ethernet.

The limitation comes from your choice of radios. Some radio transceivers only support one master on the network. Others work in a peer-to-peer fashion allowing any station to initiate a conversation. In the former, you can still setup any of your stations to generate a master poll but this all depends on how 'mastering' is implemented in the device in question.

I know of some native Modbus PLCs that are setup as 'masters' in the logic. As a result, you can have multiple of these PLCs polling each other, in effect having a multiple masters in your Modbus network. Even with radios that do not support multiple mastering, these PLCs were still able to poll each other.

Klaus

 

[xJAyx]

Thanks Klaus,
I have actually long since solved this problem, but hopefully other people can benefeit from the later posts.

What I used was Modbus TCP using MDS iNet radios transmitting to a MDS 5 series redundant backbone ring. I used all SCADAPack RTUs and front end and the multiple master, unsolicite dmessaging, and peer to peer communications is working like a charm, it turned out I didn't use the time stamping, but I'm sure I could have programmed something if I wanted to. The SCADAPack MSIP function allows pretty much all of this functionality, as long as the correct timing and error checking is set up to ensure there is not too many connections to one RTU.
Using the ethernet radios makes things so easy, doing this over serial would have been possible, but a fair bit more work.

 

[techwizard]

Modbus uses Ascii and Modbus RTU uses binary and they both are different.However both are master/Slave but cannot coexist on the same network because a network cannot have two masters