I have an emergency problem with my network communication.

I have 4 HMI (DELTA brand) and one cpu (same brand SV type) and I want to make network to control my process line. (each HMI can send some commands to PLC)

Is it possible to use RS485 for them or not? if yes or no what is the topology.

I thought it should have one master and other must be slave just to show events or messages but I want to send commands from each HMI panel.
thanks

 

You need to make one slave out of each HMI, and setup the PLC as a master. Each HMI should communicate with the PLC, and the PLC should read and write data from the four HMIs. It's not that difficult.

leomartinezb78 [at] hotmail.com is my e-mail. If you can't do that, i could program it for you for a few dollars...

 

Theoretically, the PLC could be a Modbus RTU master and sequentially poll the HMI's (Modbus slaves) standard Modbus RTU over RS-485. However, the response to HMI generated commands is likely to be perceived as slow, because a command could be initiated on the panel just after a completed polling operation from the PLC, so that the command is not fetched until all the other HMIs have been polled. This assumes that command actions are held in buffers on the HMI, until the comm deals with the command. For instance, a push button's logic state might have to valid for a period longer than the button is pushed so that the state can be communicated to the PLC.

Under standard Modbus RTU over RS-485, with the HMIs as masters and the PLC as a slave, messages appearing at the same time will collide. Bad comm. Does the master retry? This is the no-no condition for Modbus RTU, multiple masters on the same comm link.

However, there are a couple commercial "link moderator" boxes that take on the role of handling simultaneous requests. The manufacturer/vendors of these boxes frequent this forum and might pop up with a solution.

It's my impression, never having used it, that Modbus Plus was designed for multiple HMIs, with its ability to use a token passing scheme so that there is only one active master at a time on the comm link. Whether your brand HMI's and PLC can run Modbus Plus components is an outstanding question.

You might be able to pull it off with Modbus TCP, with the HMIs as Modbus masters and the PLC as a Modbus slave. The traffic through an ethernet switch to one particular destination port can be sequenced, but it isn't clear how the PLC would response to closely timed, multiple messages.

However, I have to assume that Modbus RTU slave devices are designed with the understanding that there is sufficient time to respond to a (single) master's request, with the single bus master waiting patiently for the slave's response. Hence, I'm sure many RTU slave devices are not programmed to handle a 2nd master request while they're still processing the first request.

So, would your PLC modbus RTU slave process the first request, store the 2nd, then process the 2nd? Or abort the first, execute the 2nd? Or choke on the first being disturbed by the 2nd? Or lockup because it causes a fault never considered? I don't know.

 

Your HMI must take care for this.
Our http://pvbrowser.org
does take care.

The HMI is client/server much like a web browser.
The data acquisition is done by a background program (daemon) which talks to the fieldbus (Modbus RTU for example) and writes the variables to a shared memory.

Thus there is only 1 program (daemon) talking over RS485 and you can have any number of clients showing the HMI.

See:
http://pvbrowser.de/pvbrowser/pic/prinzip.png

 

Have you considered using OPC? An OPC-server could communicate with the PLC using RS485 (modbus or other). Then each HMI, as many as you like, get there data using OPC from the OPC server.

There are OPC servers available for many PLC's and other systems. Look, for example, at Matrikon.

 

Hello Morteza,

What's the protocol used over the RS485? There are some protocols that support Token Ring (Profibus FMS). Anyways, using OPC is faster for you. You can use an OPC Server for you PLC that will connect to your HMI.

Search for an OPC Server for your PLC by protocol here:

http://is.gd/cwXUH

Hope this was helpful

Cheers,
Wassim Daoud
Global Solutions Architect
www.MatrikonOPC.com

 

Hi,

How you solved this communication? i have the same problem, can you help me?

thanks.