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twetechnics
January 12th, 2011, 05:12 PM
MY customer's plant has a Main PLC (SLC 5/05) which communicates with one CompactLogix and three other SLC 5/05's. There are Six (6) PC's with OPC servers communicating (Read/Write) with the main PLC. All of these devices share an ethernet newtork and it is VERY SLOW. The question is what is the best practice for a network such as this. Should we add another ethernet card to the Main PLC and have that communicate with the OPC servers? Should we use a different communications method for the PLC Only Network? The I/O count on the network is approximately 300 Digital I/O and 200 Analog I/O plus another 500 registers.

OkiePC
January 12th, 2011, 06:32 PM
Welcome to the forum.

I think you may need to provide more information to get good responses.

What part(s) of the communication appear to be very slow?

Are the inter-PLC communications done with multiple MSG instructions?

Can you provide a sketch of how the network is laid out including switches/routers?

Paul

sthompson
January 12th, 2011, 07:06 PM
Ethernet should not be that slow, however, there are limitations on how many Ethernet connections the SLC5/05 will support. The SLC Communications are done by the PLC during the Housekeeping part of the cycle, so it is limited to how many messages it can process each cycle. Here is the Ethernet Manual;

http://literature.rockwellautomation.com/idc/groups/literature/documents/qs/1747-qs001_-en-p.pdf

Are your messages on some kind of Interval Timer? What is the update frequency of the OPC Servers? You may be able to use a SVC Instruction to help with the SLC response time, however, bear in mind that this doess affect your scan time. It is in Chapter 12 of the Instruction Reference Manual;

http://literature.rockwellautomation.com/idc/groups/literature/documents/rm/1747-rm001_-en-p.pdf


There is also information on the Ethernet Communications in the Hardware Manual in Appendix G;

http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1747-um011_-en-p.pdf

Stu....

The Plc Kid
January 12th, 2011, 07:32 PM
AS has already been posted too many message instructions will clog slc procesors on ethernet real fast.

What links the network? Make sure you have swithces and not hubs.

Check the duplex and link speed of all devices as they need to be the same for the best performance. Make usre your devices are all set for 100 T and none are at 10T

Check the ip addresses and subnet masks of all devices and make sure they are on the same logical ethernet network and not together in a SuperNet

Binaural
January 12th, 2011, 08:02 PM
Another trick I've used in the past with SLCs - XOR the memory area you transfer to another PLC to see if any bits have changed. If changed, then send, otherwise there's no need to clog the processor.

sthompson
January 12th, 2011, 08:14 PM
Another trick I've used in the past with SLCs - XOR the memory area you transfer to another PLC to see if any bits have changed. If changed, then send, otherwise there's no need to clog the processor.

I like that idea, I have some pretty heavy traffic on my DH+ Network, and although I have them on Trigger Timers, in many cases the Data doesn't change for hours. I think I will implement that or a variation on that depending on the Processor.

Kudos! :bow::bow:
:beerchug:

Stu.....

surferb
January 14th, 2011, 02:18 AM
What kind of hub/switch are you using?