Forums.MrPLC.com: Compact Logix/modbus Protocol - Forums.MrPLC.com

Jump to content

Page 1 of 1
  • You cannot start a new topic
  • You cannot reply to this topic

Compact Logix/modbus Protocol Rate Topic: -----

#1
User is offline   plcnovice 

  • Sparky
  • PipPipPip
  • Group: MrPLC Member
  • Posts: 12
  • Joined: 23-February 06
  • Country:Canada
    Canada
Hi There
This is my first time on this forum and i am throwing my first post. In fact the reason behind this is that i am pretty new to the PLC programming world. Alright lets get back to business. I have a project to do, whose description is as follows.

We need to make compactLogic processor communicate with the modbus devices over the modbus protocol. The thing is Compact Logix does not support Modbus protocol without putting a Prosoft Hardware module (MVI69-MCM). My job is to communicate compact logix PLC with the modbus devices without the use of any hardware module. Such as writing the read and write functions for modbus in ladder logic program something like that!!!!! I have no idea how to do it. Any suggestion or ideas will be appreciated. Thanks in advance.
0

#2
User is offline   Eddie Willers 

  • Automation Advisor
  • PipPipPip
  • Group: MrPLC Member
  • Posts: 226
  • Joined: 03-January 02
  • Country:United States
    United States
A-B engineers wrote some example code to do exactly what you describe; perform Modbus RTU commands using the ASCII mode of the CompactLogix serial port. It is included in the sample programs that come with RSLogix 5000.

Unfortunately, this code is very complicated, and I've been told by some who have tried to use it that there are deficiencies in it. Therefore, I would not recommend it for a beginner.

The MVI69-MCM is really the most straightforward way to get Modbus RTU functionality on a CompactLogix controller. Other methods include Modbus/DeviceNet gateways from Western Reserve Controls, Hilscher, and HMS Fieldbus, as well as EtherNet/IP to Modbus gateways from Real Time Automation.

There is even a Modbus RTU/DF1 converter that A-B sells to connect to PowerFlex 4/40 series drives.

There's always a price for functionality; it's not realistic to say "I have $0 and I need a function that's not built in". If this is a one-off project you're going to spend many times more in labor than you would have in hardware.

Tell us more about the Modbus devices you need to communicate with.
Forum posts are not professional advice, are for educational and discussion purposes only, and are not a substitute for proper consultation with a qualified engineer.
0

#3
User is offline   TechJunki 

  • Sparky
  • PipPipPip
  • Group: MrPLC Member
  • Posts: 272
  • Joined: 27-February 05
  • Location:Midwest
  • Country:United States
    United States
I glanced throught the sample program...This Program is definitely not for beginners.

Although it appears this may be possible using the serial port I agree with Eddie, the time spent would probably not be justified. I have used the ProSoft module with great success. With it I had my ModBus network fully functional within a couple of days.

Here is the sample code for Modbus Master from Rockwell...

Attached File(s)


This post has been edited by TechJunki: 24 February 2006 - 04:13 AM

0

#4
User is offline   plcnovice 

  • Sparky
  • PipPipPip
  • Group: MrPLC Member
  • Posts: 12
  • Joined: 23-February 06
  • Country:Canada
    Canada

View PostEddie Willers, on Feb 24 2006, 12:46 AM, said:

A-B engineers wrote some example code to do exactly what you describe; perform Modbus RTU commands using the ASCII mode of the CompactLogix serial port. It is included in the sample programs that come with RSLogix 5000.

Unfortunately, this code is very complicated, and I've been told by some who have tried to use it that there are deficiencies in it. Therefore, I would not recommend it for a beginner.

The MVI69-MCM is really the most straightforward way to get Modbus RTU functionality on a CompactLogix controller. Other methods include Modbus/DeviceNet gateways from Western Reserve Controls, Hilscher, and HMS Fieldbus, as well as EtherNet/IP to Modbus gateways from Real Time Automation.

There is even a Modbus RTU/DF1 converter that A-B sells to connect to PowerFlex 4/40 series drives.

There's always a price for functionality; it's not realistic to say "I have $0 and I need a function that's not built in". If this is a one-off project you're going to spend many times more in labor than you would have in hardware.

Tell us more about the Modbus devices you need to communicate with.


Hi Eddie
Well thank you very much for your expert advice. Are you talking about "Sample logic using the SLogix 5000 sample program. You are certainly correct that why would we need to spend time on it why no just use prosoft module. The thing is we have huge projects going on like this. and that is why for every project adding a prosoft module will cost us money, therefore to reduce the project cost and due to the regularity of these projects we decide not to use Prosoft modules.

As you said there is some definicies in the program which i think you mean to say bugs which will prevent the program to run properly right? how could i be able to remove the bugs? if possible.........

As for as the modbus devices which i need to communicate are concerned, basically its a engine system module in a compressor which only works as a modbus salve on the modbus network. and we use these modules extensively in our projects...so thats a primary reason that i put up this question.

Eddie i would really appreciate you if you help me out in figuring out this issue..........thanks in advance

PLCNOVICE
0

#5
User is offline   paulengr 

  • Propeller Head
  • PipPipPipPipPipPip
  • Group: MrPLC Member
  • Posts: 1,416
  • Joined: 27-March 05
  • Gender:Male
  • Location:North Carolina
  • Country:United States
    United States
I've taken a different (but still hardware) route with respect to Modbus and AB PLC's.

Everything in our shop is PLC-5's (almost 2 dozen) except one SLC and 2 Micrologix. The biggest attraction is that Modbus is much more widely supported and much less expensive than AB's I/O offerings.

So we're still testing but we went with the Digi One IAP card. That thing has solved more problems than we have tapped into so far. It has already bridged our aging DH+ network (who wants to spend almost $1500 per PLC using the AB way to add Ethernet when I can outright buy the whole PLC for about the same amount of money...). We're now testing Modbus/TCP bridged over using MSG blocks on an Ethernetted processor using I/O from Acromag. Reason being that the cost difference between full blown Ethernet and using Modbus/RTU was small. But the same IAP card could trivially bridge Modbus/RTU with the same configuration.

I agree that in theory you could write your own Modbus/RTU or Modbus/ASCII protocols using the serial ports. What a pain. However, in Ethernet, there is no equivalent "give me a raw TCP session" type interface.
0

Share this topic:


Page 1 of 1
  • You cannot start a new topic
  • You cannot reply to this topic

1 User(s) are reading this topic
0 members, 1 guests, 0 anonymous users