Let me start by giving my background of Modbus ----



NONE!!!!!!



My customer's plant is entirely AB. We are in the process of putting in a new production line, which includes a new spiral freezer. The freezer is controlled a Compact Logix L42. The compressors and chillers are all controlled by proprietary micro processors, which are capable of talking Modbus-TCP. The customer would like the alarms for the Micro processors to be integrated into the plants building control/utilities HMI, which is an RSView32 application. The Compact Logix will be on the same Ethernet network as the RSView application, so I am planning on using that to transfer the data.



I have purchased the following card from Prosoft.

http://prosoft-technology.com/content/view/full/6458



From what I have read on this module, it should work for my application. My question pertains to the network infrastructure. Does Modbus TCP require any special hardware or wiring, or is it very similar to AB's Ethernet/IP? Do I need a managed switch, or will any switch work? Does CAT6-E work for media? My network will consist of the Prosoft card and 4 micro processors. I am planning on setting this up as its own network, keeping it segregated from my Ethernet/IP network.



Any advice will be greatly appreciated.

Modbus/TCP is nothing like Ethernet/IP.

Modbus/TCP has more in common with DF-1. Essentially, Modbus/TCP is nothing more than Modbus/RTU encapsulated into a TCP packet stream.

You really should have managed switches (and set up a lot of things) to avoid problems with Ethernet/IP. The biggest problem with Ethernet/IP is that it uses multicasted UDP for data transfers. If you don't use managed switches, then Ethernet/IP turns into broadcast packets and depending on how much I/O you have, quickly nukes your I/O devices and/or the network with broadcast chatter going everywhere. You can turn off multicasting but that's not the default.

Modbus/TCP is pure unicast. Hence it is never a problem.

Modbus/TCP is also a published and truly open standard, one that you don't have to pay $5,000 even to get a copy of the standard, and you don't have to pony up additional money just to market your device. The protocol itself is so simple that you can throw together a driver for it in a matter of minutes if you are a good programmer. Ethernet/IP in contrast is very complicated and the standard is just as "open" as OPC...which is to say, it's not really.

You can use any Ethernet cable, switches, etc., with Modbus/TCP. It's a pure request/response protocol just like DF-1 or similar protocols. It's also so darned simple to write drivers for it that it's almost uncanny. Too bad AB doesn't give you a way to do "raw TCP" in their PLC's. You could have also used a Digi One IAP which is a much simpler and less expensive alternative to Prosoft modules.

There's also nothing wrong with having Ethernet/IP and Modbus/TCP on the same network. Heck, you can even integrate your office network with both IF you are very, very careful about how you set up your managed switches (and you better use managed switches). Since Modbus/TCP is just an ordinary unicast stream you don't have to worry about all the Ethernet/IP'isms. The biggest concern that you should have is with response time and bandwidth issues...which are very easily solved. It shouldn't be any bigger of a concern than an Ethernet-HMI.

That's not to say that Ethernet/IP doesn't have some huge advantages over Modbus/TCP. It's just that you pay dearly for the extra features in terms of added complexity and added protocol bureacracy.

As to CAT-6, this is an orphan standard. CAT 5E is the "enhanced" version of CAT 5. It basically fixes some specification issues. CAT 5E cable has a 100 MHz bandwidth. CAT 6 cable has a 250 MHz bandwidth. Because of spectral shaping and using all 4 pairs, gigabit Ethernet works just fine with CAT 5E cable. There is no performance advantage with CAT 6. Under the 10 gigabit standard, you can only go a few meters with CAT 5E, and CAT 6 is limited to approximately the same distance. So that's why CAT 6 is an orphan...there is no corresponding Ethernet standard capable of truly taking advantage of the additional bandwidth that the cable standard offers. In order to support 10 gigabits at 100 meters, the RJ-45 connector has to be changed and additional efforts have to be devoted towards handling near/far end crosstalk. Simply reducing the jacket thickness and introducing some additional twisting and shielding (what was done for CAT 6) is not enough of an improvement. So once a CAT 7 standard comes out, it won't be using RJ-45's, and the cable is potentially not even going to be twisted pair.

So my advice is to buy CAT 6 if/when it is equal to or less expensive than CAT 5E. About the only true advantage that you get from it is that since the cable is more rounded, it can potentially be easier to pull. The increased bandwidth is lost because there's nothing out there to take advantage of it.