What is difference between Modbus Comm and Profi bus? what is the comm that is accepted by A-series Mitsbushi PLC? Any website to have a clearcut information are welcomed.

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Hello ganesh;

You can look at these different sites:
Profibus:
http://www.profibus.com/pb/technology/description/

Modbus:
http://modbus.org/tech.php

Bus comparison file:
http://ourworld.cs.com/rahulsebos/buscomparison.pdf

Documentation links on both protocols:
http://www.romilly.co.uk/profibus.htm

Mitubishi PLCs:
http://www.meau.com/eprise/main/sites/public/PRODUCTS/Programmable_Logic_Controllers/default
http://www.mitsubishi-automation.co.uk/products/networks_profibus.html

Basically Profibus-DP and Modbus RTU are protocols used to exchange data between industrial controllers and field instruments.
Modbus is older (1979; by Modicon), slower (often 9600 Baud) and cheaper; it started without a standards organization (now taken by modbud.org) so implantations varied a little between vendors. There alre other flavors of Modbus: Modbus ASCII, Modbus+, Modbus/TCP. Modbus RTU's physical layer support includes RS-232, RS-422 and RS-485. Basically all registers transferred on a Modbus exchange are 16-bit integer (floating-point values are transferred over 2 registers) and use the original Modicon register addressing (1000x for input coils, 3000x for holding registers…). The Modbus master is assigned jobs to communicate with each slave, and uses Modbus functions to request different types and number of registers inside each job.

Profibus-DP is one variety of Profibus protocols (others include FMS and PA) designed for fast access to remote I/O. It can go up to 12MB, and basically uses the RS-485 2-wire physical layer specifications. Up to 32 slaves can be setup in a single segment, up to 125 slaves using repeaters. All exchanges are configured once, in the master, and all details of number and size of data exchange with each slave is determined at that time. Cyclic exchange is very reliable and deterministic; acyclic exhanges allow good diagnostics of each node, and special profiles to be added to the original specification (DPV1, DPV2, Profisafe, Profidrive, IRT...).

Read up on the documentation above, it can be a good start.

Hope this helps,
Daniel Chartier

 

Acromag offers videos and white papers on Profibus and Modbus. Here is the link to find them:
http://www.acromag.com/supi_tutorials.cfm

I searched Mitsubishi's website for "A Series" and found a motion
controller. It looks like they support CC Link Protocol. To go deeper on their website, you'll need to know their model number and search their documentation. Go to their website shown below and then click on Service/Download on the bottom left side. You should be able to use their search tools.

http://www.mitsubishi-automation.com/iQ

Kind Regards,

Donald Lupo

 

Modbus runs on RS-485 or Ethernet via a TCP/IP wrapper (also RS-232, 422 etc.). Modbus supports ASCII mode or RTU and up to 250 slaves on each network (like all field buses, best to derate loading by 50% for acceptable performance). Google for more details!

Modbus Plus uses a peer-to-peer communications topology but is not as open as plain Modbus.

PROFIBUS-DP is another field bus that runs on RS-485 and supports up to 126 devices. Speeds can range up to 12 Mbps, this depends on device capability and trunk length. PROFIBUS is an open bus like Modbus. For Ethernet you would use PROFInet instead (not just PROFIBUS-DP in a TCP/IP wrapper). Google for more details!

They are both capable field buses. I have used both - usually Modbus for things like tank gauging, instruments, automatic valves in large processes where it was dictated by the customer. I've used way more PROFIBUS in everything from machines to processes - you also have the option of PROFIBUS-PA and hazardous environments.

If I was starting out fresh I would probably defer to PROFIBUS given the choice between the two. This could elicit some "flames" as a lot of people would swear by Modbus.

As far as your Mitsubishi PLC I know many Mitsubishi PLCs support PROFIBUS. I don't recall if they support Modbus. I personally don't care for Mitsubishi PLCs, but it has been a few years since I used one, so I'm sure they have improved since then.

I'd take a strong look at Rockwell Automation or Siemens as PLC vendors. I think they are both just better than Mitsubishi.