Hi,

I've new to field bus stuff. I've been reading stuff for the last few days but still have a few hopefully easy question w.r.t modbus and profibus.

1. Both modbus and profibus are master/slave architect. So a slave cannot initiate a transaction to the master, correct?

2. We have a device with RS232 connector. What's the easiest way to get this onto the modbus/profibus network? I'd presume it'll be a hardware solution. Any recommendations?

3. From what I can tell, for a device to be integrated into a new network, firmware will have to be changed for that network. E.g. each modbus master would have different notion which register is which. Is this correct?

Thanks,

 

> 1. Both modbus and profibus are master/slave architect. So a slave
> cannot initiate a transaction to the master, correct?

Correct, although I believe Profibus has some sort of mechanism that allows more than one master to access the same slaves over the same network.

> 2. We have a device with RS232 connector. What's the easiest way to get
> this onto the modbus/profibus network? I'd presume it'll be a hardware
> solution. Any recommendations?

Well, it might be running Modbus already. Modbus and RS232 are different layers, like bread and jam. Not interchangeable, not incompatible, just addressing different issues. Also, if you're not certain it's RS232, don't assume that because the connector is a DB9. Same story -- tons of protocols can be wired to a DB9, and RS232 can be wired to whatever connector you'd like. Profibus uses a DB9 itself, for example.

But yes, if your port is RS232, and not Modbus, then you're going to need some sort of bridging device to connect it to a Modbus or Profibus network. In fact, if you have an existing Modbus "network" and you want to add a new node, that implies it has multiple slaves or is "multi-drop" in which case it's probably RS485. In that case, even if your RS232 port IS Modbus, you'll need some hardware to interconnect the two.

> 3. From what I can tell, for a device to be integrated into a new network,
> firmware will have to be changed for that network. E.g. each modbus master
> would have different notion which register is which. Is this correct?

No, the layout of the data is (usually) determined by the manufacturer of the slave device. The master will have some means for you to program it (probably not firmware) as to what slave node numbers and register addresses within individual nodes it needs to read from and write to.

 

> 1. Both modbus and profibus are master/slave architect. So a slave
> cannot initiate a transaction to the master, correct?

Modbus: 1 master N slaves
Profibus: N master M slaves
Yes, only a master can initiate a transaction
> 2. We have a device with RS232 connector. What's the easiest way to get
> this onto the modbus/profibus network? I'd presume it'll be a hardware
> solution. Any recommendations?

You need a RS232/RS485 adapter.
With RS485 you can implement a serial bus.
http://en.wikipedia.org/wiki/EIA-485

>3. From what I can tell, for a device
>to be integrated into a new network,
>firmware will have to be changed for
>that network. E.g. each modbus master
>would have different notion which
>register is which. Is this correct?
The firmware does not need to be changed.
You will have to configure which variable is mapped to which register.

 

1) Correct for Modbus. Slave cannot initiate transaction.

2) "A device with RS-232" is a meaningless statement. What communications protocol does the device use? Modbus? Profibus DP? Epson serial printer?

You're assumption about an 'easy hardware solution' is not correct. Communications needs a hardware layer, a comm protocol and a software/firmware app to interpret the data. Converting hardware layers (serial to ethernet for example) that use a common protocol like Modbus is a relatively easy doable task, compared to trying to write data over RS-232 to a device that only wants to transmit its data to a serial printer.

3) What kind of device? What kind of network? What's the difference between old and new network?
What kind of changes to firmware?

 

As always, more information is better. I'll try to answer some of your questions, but I'm afraid the answers aren't especially helpful.

"So a slave cannot initiate a transaction to the master, correct?"

Correct. Always correct in the case of Profibus. However, things are a little trickier with Modbus. While most Modbus devices are either a master or a slave, nothing stops a Modbus device from being BOTH. So it's true that a slave can't initiate a transaction, but some devices can switch roles on a whim.

"What's the easiest way to get [an RS-232 device] onto the modbus/profibus network?"

There are lots of solutions for this out there. Anybus, Moxa, Digi, Hilscher, etc. all have converters. On top of that many I/O vendors have special "slice" modules for RS-232. Wago (750), Beckhoff, B&R (X20, no Modbus), Allen-Bradley (Point I/O, no Modbus), and probably many more.

As several other people have pointed out, saying "RS-232" is not really enough. If it's generic ASCII communication, great. If it's some bizarre protocol that's proprietary and/or undocumented the fact that you can "talk" won't help. It'll be like using a telephone to call another country. Sure, you're getting data back and forth... but neither of you can understand the others data.

"For a device to be integrated into a new network, firmware will have to be changed for that network."

No. Changing firmware is a big deal. However, configuration settings probably have to be changed. For Profibus, this is a semi-big deal. You have to have special software (provided by the vendor of the master), and when you're ready to go live you'll have to stop the network, at least for a few seconds. Modbus, on the other hand, doesn't have that problem... because it's NOT A NETWORK. It's purely point to point. If you add a Modbus device to a "network," you have to have another RS-232 port on your master.

"I've new to field bus stuff... a few hopefully easy question w.r.t modbus and profibus."

Profibus and Modbus are so fundamentally different that asking about them at the same time makes no sense. It's like asking about motorcycles and airplanes. Sure, they both move you and both have engines, but they're used for completely different purposes. Profibus is a fieldbus, or industrial network. Modbus is a protocol, usually sent over RS-232 serial.

By the way, this is an odd time in history to be researching Modbus and Profibus. Modbus is rapidly disappearing. Profibus is going to start fading soon. At this point, they are akin to ISA and PCI slots on motherboards. All of the exciting stuff is going on with Ethernet based industrial networks, including Modbus/TCP and Profinet.

What is it, exactly, that you are trying to accomplish? Because while your specific questions are easy enough to answer, I don't feel like they're helpful in moving towards an actual GOAL. Do you have some small task to accomplish? Have you just been made head engineer at a petrochemical plant? Are you in IT? Why Modbus and Profibus?

And, of course, long-time readers know what I'll say next; where are you located? Modbus is pretty universal, and Profibus covers most of the world. But in North America and Japan, Profibus is not the dominant fieldbus.

I'd like to help more, but I just don't have enough information to say much that's useful. (I realize that it's ironic to say that after a ridiculously long post.)

-James Ingraham
Sage Automation, Inc.

 

Thanks for the long reply.

I work for a company that builds water analysers. And customers want to put our analyser into modbus/profibus network. As simple as that. A lot of our customers are in pharma industry and AFAIK, this is a slow-moving industry. So I don't know how widely spread MODBUS/TCP and Profinet is.

Isn't MODBUS/TCP simply MODBUS data transmitted over TCP?

BTW, is there email notification on this forum?

 

Hi,

Your friendly local moderator here:

> BTW, is there email notification on this forum?

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Regards,
Peg Ferraro

 

"customers want to put our [water] analyser into modbus/profibus network."

Well, that clears things up a bit. Now the hard part is figuring out how to get your analyzer to talk to a serial port or Profibus ASIC. I'm afraid I'm a little out of my depth on that one.

"Isn't MODBUS/TCP simply MODBUS data transmitted over TCP?"

Essentially, yes, but there are some minor differences. Notably, Modbus/TCP does not include a CRC, as the Ethernet and TCP layers already handle that.

-James Ingraham
Sage Automation, Inc.