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Hello, everybody!


And now about our problem.. We have a net of 3 PLC's and monitoring computer with SCADA. All the PLC's controll various processes in their controll rooms and exchanging data with SCADA computer. All PLC's connected with each other CANbus net and because 1st controll room is near, we used serial connection to connect it to SCADA computer, where OPC server is ran and wonderware SCADA. In attached picture you can see topology we made. All cables are 5e cat. FTP cables (it was not our choise-other company made installation). In PLC's with ID 1 and 3 we put EOL resistors on CANbus connection. On each PLC's power-up we store id numbers to SI8 and initialize CANbus (ISC) with 100kb baud rate.

PROBLEM: at first and for some time thereis no problem: data exchanging goes fast and good (data exchange is made every 2sec). But after some time, computer looses PLC of control room 3. Sometimes data exchange is restored (after some time) sometimes not. Sometimes computer looses PLC of control room 1 or 2.

At first we thought it is because of cable length and changed the CANbus baud rate to 50kb (to all PLC's). But problem still occurs..

Maybe it's because there are used FTP cables?..

Please, it would be nice if someone could share his oppinion about CANbus network. THANK YOU!

post-233-050215000 1294084996_thumb.jpg

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You've got to use CANbus cable, as two of the conductors are designed to carry the bus power, and the other two are twisted and shielded. Cat 5 isn't the same.

Call your cable supplier and tell them you want some Allen Bradley DeviceNet cable - the "thick" stuff.

Belden cable

I'm not sure if the star topology will work either; it's not real clear in the Unitronics documentation. One of the home boys will have to answer this.


Joe T.

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Dear Vadimas,

Sorry to say, but this system is built in the worst possible way!

1. CANbus network is "pure" dayzy chain. This means the cable needs to pass from one unit to another. You did it with "star" topology, wich is definitelly wrong!

2. CANbus is very sensitive to the cable. You MUST use the right cable - DeviceNet Thick one (see attachment).

I see two options:

1. To totaly redesign your network. In this case please follow strictly the instructrions in VisiLogic's help.

2. To switch to Ethernet. In this case you will need to add Ethernet card to the controllers, switch/hub somewhere and to rewrite the communication part of the application, but will not need to change the cables.


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Good day, Emil,

The problem is that , as I mentioned, the cables were installed not by us. That happened because company didn't know what protocol will be used. There are spare cables installed and there is possibility to get daisy chain. I think we will try it because to change cables is almost imposible. By the way there are ETHERNET cards installed in all PLC's, but we are using them for communication with other devices. If there will be problems we will install switches/hubs and will be using ETHERNET.

As Joe Tauser said: it's not really clear in the Unitronics documentation what topology should be used.. There's shown parallel connection of all devices (which can be in star topology) and no word mentioned about daisy chain..

Thank you for your quick help!


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Hi VS,

I'm pretty sure you know, but anyway - in Ethernet you can open 4 sockets and - as result - to communicate with 4 devices at a time. Are you already using all 4 sockets?

If the controllers are already connected in Ethernet LAN, there is a great chance that you don't need any changes in hardware/wiring. Probably only programming. Take a look on the funtions TCP Raw and UDP Raw. Those are Unitronics Ethernet Multimaster protocols and it seems at least you will not need to change all the conception of data flow in your project.

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