Modbus programming

thejoker954

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Hi all, I'm currently using a IDEC Pentra FC5A and need to use it as a modbus slave. I am having some trouble figuring out how to do this, I can make it a master no problem but I can't seem to make it a slave, does anyone know where I can find some programming examples for a FC5A modbus slave or knows how to program the transmissions? this is my first time doing anything with modbus and Idec's documentation really lacks any usefull info on programming a slave.
 
Just from a quick Googling of 'IDEC Pentra FC5A' and 'MODBUS' I found an intro paper from B&B electronics. It looks like the only setup on a slave is to set its address. Otherwise the master does all the work.
 
Manual is at
http://www.bb-elec.com/bb-elec/literature/IDEC/Pentra Modbus Training_1-12-07.pdf

Slave settings:
1eb6f4.jpg


Dan
 
Set up the Slave first, then switch to S7

I am not using IDEC , I am using S7. There is a lot of useful information in this document, but the method of setting the slave address does not appear to be relevant to S7. Or I am missing something.


You need to set up the slave device (Modbus Slave ID Number is always relevant) first in whatever software is used (see page 6 of document from B+B), then switch to the S7 master and give it a go.
 
You need to set up the slave device (Modbus Slave ID Number is always relevant) first in whatever software is used (see page 6 of document from B+B), then switch to the S7 master and give it a go.

First , thanks for the help.

I assume the IDEC software is free somewhere and you propose to work around Siemens just to change the slave address. While this gets the job done, it sort of lets Siemens off the hook.

A part of me realy wants to force them to face thier own documentation and software with me. I am beyond just trying to work something out. I want someone at Siemens to receive my feedback and possibly add it to others and actualy bring a better product to us or at least a revised manual.

The Siemens software should be able to change the slave address.

If not, it needs to be fixed so it can.

And the documentation needs to be more concise and more clear.

Hopefully, after I get to a good point , I can organize everything I have been through with this S7 adventure into a tutorial or maybe even a book.

A book , unlike Hern Berger's book and the Siemens docs, that is easy to follow.

Today a Siemens rep tells me I need the dongle and a license to use the CP341 to communicate as a Modbus slave.

Then he says, "I do not believe the RS232 CP 341 alone will communicate Modbus"

Not sure what he means by this.

I am still digging and will return here for suggestions and to update if I find more information.

Thanks again.
 
Just the facts maam.

Just from a quick Googling of 'IDEC Pentra FC5A' and 'MODBUS' I found an intro paper from B&B electronics. It looks like the only setup on a slave is to set its address. Otherwise the master does all the work.

This seems to be at odds with I find in the example. Granted I may be missreading everything.

It seems plausible to me that the Master would request data from a certain slave address and that the Master would request certain data.

Next the slave of the correct address, would have to organize the data and put it into the correct memory location or DB and Send it.

It also seems plausible that even though the slave is subordinate to the master, the slave may require some information from the Master which would imply a request of information.

In the example, we have send and receive blocks detailed.

Right now I have not seen Modbus work so I am in the dark.

It also seems plausible that the slave would have the send data all organized and ready to go all the time being updated and just waiting for the Send command.

If any of this is true, then you need something on the slave to organize the data and recognise the Send command.

Of course, it is also possible that the Master just takes the data directly from the slave memory locations or DB, but it doesn't seem like this is what I am reading.

Another possiblity is that both / all of these are possible and Siemens just picked the most complicated and least likely scenario as the example for new users.
 
Thanks for your patience, I'm sorry I don't know Siemens, I really can't offer you any help on that. However, I think we need to take a step back. Modbus is arguably the first feidlbus, and it is not a Siemens standard, it was developed by Modicon back in the day. It is a pretty basic command / response serial protocol. A Modbus Master sends a request that contains a slave ID, all the slaves receive the request, and if the slaves ID number matches, it looks at the command and responds to it. If the slave ID doesn't match, the command is ignored.

So, the short version is, on the IDEC slave device, you have to set up the Slave ID with whatever tools IDEC offers before it will be able to answer any Modbus queries from the Master device, regardless of the vendor of the master, in this case the Seiemens.
 

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