Devicenet, Modbus, ETC. ??? 2

[milmat1]

Hello All:
I have been programming for about 2 years now, PLC's, HMI's, Motion, All using GE-Fanuc products. However, I have a terrible week spot. And that is understanding all the different types of Buss' that are out there for data transfer. I have used Ethernet to some degree, And have set up some remote I/O with versamax PLC's talking to an RX3i Main Rack over Ethernet.
But beyond that I am totally lost ! With all that I have read on the subject of Buss' I am just becoming more confused. Could someone give me just an overview of the Structure of some of the more popular buss' like devicenet, modbus etc...

Any info will be helpfull as I am lost here....

THANKS

Regards,
Matt

 

[TWControls]

Here is a site about Modbus, which I consider an older protocol (I may get reamed on that one)

http://www.modbus.org/

As far as Devicenet check out

http://www.odva.org

But this is more of general info. A little more about your desired network may help. Some are designed for large amounts o information and some are designed for fast I/O response

----------------------------------
TW Controls -

http://www.twcontrols.com

 

[controlsdude]

Controlnet
called deterministic or broadcast type network
5msec nut adder on any remote drops
can schedule drops till your network is at 100% usage
RG6 cable
Not sure on baud rate, but used for any high speed io

Profibus (or can bus)
variable data speeds (higher speed shorter distance on cable, token ring, but 10x the speed of devicenet)
Purple cable spec? (barney cable, nickname)
if one device goes down any device comm link downstream stays up and running.


Devicenet (similar to Profibus)
token ring
any network where product not going faster than 100fpm
4 wire system flat cable or 4 pin cable
if one device goes down any device comm link downstream stays up and running.

Fieldbus
for runner of profibus
used in europe or germany
if one device goes down any device comm link downstream goes down with it.
cable spec less stringent than profibus

DH+
token ring
slow but stable
blue hose cable spec (remote io)
if one device goes down any device comm link downstream stays up and running.

biggest problem with distributed networks is the implementation of estops. Where ever you have outputs you need to bring out a wire to interupt output power. But this is being overcome by so called "safe network" permentation of the above networks. Not sure what this means by safe network.

every comm network except ethernet usually has terminating resistors
every network has a maximum distance based on a baud rate and amount of devices on network.
every network is based on a particular cable spec.
every network usually has a master scanner module except ethernet and dh+, not sure about modbus+(similar to dh+?)
Master scanner - Controlnet this is called the keeper module (later revisions more controlnet cards can be the keeper.)

I probably misspoke on some it but thats all i remember.

 

[milmat1]

The links are helpfull, As well as Controilsdude's explainations. I may eventually grasp this thing..LOL..

Thanks All !!

MATT

Regards,
Matt

 

[CanadianTaff]

I wouldn't say DeviceNet is similar to Profibus at all.

Profius has a few forms, most popular being DP you can also have FMP I believe its called.

Profibus is more comparable with ControlNet.

The lower form of bus under Profibus, more similar to DeviceNet is Asi. Asi links sensors, pushbuttons etc at a lower level. The nice thing with Asi is the fact you can run it together with power cables with no adverse effect.

DeviceNet cabling issues are reknown.

 

[jraef]

I like controlsdude's compendium, but I also disagree with the comparison of DeviceNet with ProfiBus, especially in that my experience with DeviceNet is contrary to the statement "if one device goes down any device comm link downstream stays up and running." While this may be accurate, it is not the entire picture. My experience is that a lot of DeviceNet devices suffer from the flaw in the overall system design that the CANBus chip set is powered by the bus. So if the NETWORK goes down, so does the device. In a sensor application which is reliant upon the PLC network anyway, the point is moot. But in applications such as Smart MCCs, where the DeviceNet capable device is often an Overload Relay, I have had it happen where the Overload Relay (AB-E3 Plus) ceased to function as an Overload when the DeviceNet cable goes TU! That to me is a major flaw.

ProfiBus and in fact no other industrial network I know of has that flaw. ASi has that flaw, but I would not call ASi a full blown industrial network. The main advantage it has however is that it can be use in a Safety environment (Asi Safe).

 

[controlsdude]

All buses have
Dip switch settings on any device in field for addressing.

I believe ethernet is the only network where you set the address in the software. Most networks force you to set the address locally by dip switches.

 

[TWControls]

"All buses have
Dip switch settings on any device in field for addressing."
This is going to be device dependent for most buses. While many Devicenet devices follow this practice some don't and it is not a requirement. I don't think any scanners do but this is more of a nice feature for the actual field devices. Many have the feature of Auto Address Recovery now. Similar to a DHCP only when the scanner sees a new device that is similar to a missing device on the network, it will assign the missing device's address to the new device and can additionally configure the new device's parameters

Many Ethernet/IP devices now have rotary switches that allow you to set the 4th octet and default to 192.168.1.XXX

----------------------------------
TW Controls -

http://www.twcontrols.com

 

[OldPfart]

Here is a link to a site on busses. There are a loit of links to other sites also.

http://ourworld.cs.com/rahulsebos/

I have recently started using a new I/O bus - CompoNet. Only available from Omron as far as I know at this stage and is administered by ODVA. Faster, deterministic (token ring) and half the price of DeviceNet. Love it!!!

 

[milmat1]

Thanks OldPfart

Great Link !!

Regards,
Matt