Siemens and Modbus RTU

I have a job to do with a Siemens S7-318/2 processor. Part of the requirement for the job is communications with several devices that have a Modbus RTU slave interface.

I have priced the Siemens card and software and got quite a shock. Certainly did not allow enough in the quote. I am therefore looking for some information on how easy/difficult Siemens Modbus is to set up? Looking to save some time to help pay for the card and software.

Any information would be appreciated.

Hello Bob;

I think it all depends on your Siemens experience. The setup package comes with dedicated functions that take care of the communication issues btween the S7 and the Modbus slaves. If you know how to "read" Siemens help files, the work is pretty straightforward. If not, you have a learning curve to follow. The programming example codes that come with the software are a very good starting point.
Actually, the hardest part was to find the adequate connectivity between the master and the slave, especially on RS-232; multiple connections are available, and standards seem to be rather elastic.
Hope this helps,
Daniel Chartier

Thanks Daniel.

I will be using a 232/485 isolated converter, Adam 4050 I think, as I will be running to 2 x 485 equipped devices.

Keep my fingers crossed - have not used Siemens for many years - last time was the first release of Windows software - was not real friendly and the support from Siemens Ozz was lousy when I had problems communicating with an expansion rack - turned out to be an "undocumented" feature of the processor that the expansion rack could be seen sometimes and sometimes not. They finished up swapping to a higher end processor that did not have the "undocumented" feature.

Will post back on how I go - will probably be a couple of months till we are commissioning.

Hello Bob;
You will be using a CP341 with the Modbus Master Loadable Drivers (this is what you need to setup a Modbus master on a S7-300 platform). You can simplify your life tremendoulsly (and I cannot insist enough on this point) if you buy a CP341 with an integrated RS-422/485 interface for your application:

CP 341-RS 232C 6ES7 341-1AH01-0AE0
​

CP 341-20mA TTY 6ES7 341-1BH01-0AE0
​

CP 341-RS 422/485 6ES7 341-1CH01-0AE0
​

Connections will be much simplified and possible failure points reduced.
Talk to us when you are ready to start your project.

Hope this helps,
Daniel Chartier

Hi Bob.

Yes the CP341 with loadable drivers is a bit expensive.
750 Euro for the CP341 + 1100 Euro for the loadable driver.

Interestingly enough the interface card that my local Siemens rep quoted is 6ES7340-1AH01-0AE0 for RS232. Further, the Siemens website suggests that I should use 6ES7340-1AH02-0AE0 - an updated firmware.

The others have been updated to version 2 as well.

Quoted price to me is $705.76 AU for the card and for the software $2,264.13 AU. A lot dearer than you pay in Europe obviously. The quoted driver part number is 6ES7870-1AA01-0YA0.

The Ozz website talks about discontinuation of the ET200 line as they have been supporting them for the legislated 10 years but time has run out. Possibly a local decision.

Fortunately I will be working on the software for the project with the hospital engineer. They have the whole hospital full of Siemens PLCs all hooked up to Citect SCADA system. All Ethernet communications as well. He has not worked with the Modbus software before either.

I normally work with the Omron CS/CJ1 series of PLCs and the serial card I normally use is a bit cheaper than the Siemens card but it has 1 x 232 and 1 x 422/485 ports on board as standard. The Modbus master/slave software is free. Siemens frighten a lot of us in Ozz with their very high pricing. The philosophy of their "own way of doing things" and always a very steep learning curve is a bit of a worry also.

Thank you both for your assistance so far. I will call in again for sure as things progress.

BobB said:

The Ozz website talks about discontinuation of the ET200 line as they have been supporting them for the legislated 10 years but time has run out. Possibly a local decision.

Click to expand...

This must be something else than the ET200S range I am referring to. ET200 is one of Siemens Automations core products.
The initial range was called ET200L, and I think it is still supported despite some modules are no longer in production. Maybe that quote refers to some of these older versions or maybe even "ET100".

Today all development is now put in ET200S and some other newer ranges. I just recieved a brochure from Siemens with all their latest stuff on ET200x and quite a bit of it is ET200S.
I have use ET200S extensively on one of my projects and I was quite satisfied with it.

Hello Jesper;

I did not know of that serial interface for the ET200S. From the online manual, it looks as it is using the same SEND and RECIEVE functions that were used by the CP341, which would make the programming rather simple. It looks like a great solution. Look into this Bob.
Daniel Chartier

You could save yourself a bucketload of cash by using the 317 instead of the 318.

You could save yourself a bucketload of cash by using the 317 instead of the 318.

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I can imagine!! I only quoted to use a 312 but the client has a lot of things he wants to do with the PLC apart from my project. It is his choice and he is going to supply the processor.

The 6ES7138-4DF11-0AB0 is a module that plugs into a ET200S rack. It contains Modbus and USS protocols allready, so there is no need to purchase and install a loadable driver

Click to expand...

The IM151 BASIC 6ES7151-1CA00-0AB0

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I gather I will need both?

ET200S is like Lego blocks. There are so many different parts that must be assembled in the right order. It can be confusing at first, and it is easy to forget something.
All in all you will need the following. It is assembled in the order as listed starting with the IM151:

IM 151 BASIC
6ES7151-1CA00-0AB0

Terminal base for power module
6ES7193-4CC20-0AA0

Powersupply module (installed in the terminal base)
6ES7138-4CA01-0AA0

Terminal base for IO (there are 5 in a pack)
6ES7193-4CA50-0AA0

CP with Modbus (installed in the terminal base)
6ES7138-4DF11-0AB0

Terminator (it must be put on as the last on the modules opposite to the IM 151). I dont remember if this one had to be purchased separately, or if it came with the IM 151.

Preconfigured Profibus cable, or 2 profibus connectors + profibus cable by the meter.

JesperMP said:

ET200S is like Lego blocks. There are so many different parts that must be assembled in the right order. It can be confusing at first, and it is easy to forget something.
All in all you will need the following. It is assembled in the order as listed starting with the IM151:

IM 151 BASIC
6ES7151-1CA00-0AB0

Terminal base for power module
6ES7193-4CC20-0AA0

Powersupply module (installed in the terminal base)
6ES7138-4CA01-0AA0

Terminal base for IO (there are 5 in a pack)
6ES7193-4CA50-0AA0

CP with Modbus (installed in the terminal base)
6ES7138-4DF11-0AB0

Terminator (it must be put on as the last on the modules opposite to the IM 151). I dont remember if this one had to be purchased separately, or if it came with the IM 151.

Preconfigured Profibus cable, or 2 profibus connectors + profibus cable by the meter.

Click to expand...

We have just done one of these to communicate with a GSM modem it works fine. No trouble programming just use the modbus slave standard FB81

Just re-read the original post, you have to get the modbus master FB