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| 12/2/2009 8:08 PM | |
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Posts: 2 Rating:
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Hi, Is my configuration as attached works ? I'm connecting OLM in Star Topology with others IM 153-1 in the same Profibus network. AttachmentMy Configuration.pdf (186 Downloads) |
| 12/3/2009 12:08 AM | |
Joined: 10/7/2005 Last visit: 12/24/2024 Posts: 3029 Rating:
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Hello ndlow it will work, but I wouldn't do it and the reasons for this are: If you link two or more OLM's via their RS485 port, you are creating a so called "OLM Star coupler". OLM Star couplers are permissible, but there are a few things to look out, most importantly: - Avoid connecting other bus particpants to the RS485 Star segment (your Topology does NOT comply with this) - Keep the RS485 Star Segment as short as possible (your Topologydoes NOT comply with this) You can find more useful info on the "issue" of OLM's Star Topologies in chapter 3.2 of the OLM manual (download link:OLM Manual) I suggest you replace theTWO G11 OLM's with ONE G12 OLMwhich will eliminate the OLM Star Topology as well as savingyou some money and attached isa pic ofthe revisedTopology for this. I hope this helps |
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| 12/4/2009 4:45 AM | |
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Joined: 10/7/2005 Last visit: 12/24/2024 Posts: 3029 Rating:
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Hello J_Bell all credit to youfor raising this point! While I probably won't be ableto give an satisfactory explanation, I shall at least try to outline some possible reasons that I can think of behind the "OLM Star Topology" oddities. Some observations to begin with: 1.) I think we can all agree that there'd be no "Star topology" issues ifndlow's Topology would be done withnormal RS485 Repeaters instead ofOLM's. 2.) We can probably also agree that usage of anOLM pair (i.e. OLM<---FO-link-->OLM) is (or should be)in principalno different than usage of one repeater. The question then is: Why does Siemens even mention the word "Star coupler" never mind having these "odd" rules in the OLM manuals concerning its implementation? Well, my take on it is thatthere are differences between usage of an OLM paircompared to a repeater. For one, an OLM introducesa Signal time delay (~6 tbit) whereas a repeaterdoes not (Siemens repeaters do nonetheless have a maximum cascading depth of 9, but this due to the introduced"inaccuracies" when regenerating the Profibus Signal. Procentec's Profihub for example are a "true" piece of the cable and have unlimited cascading depth). More importantly though (the above will have little or nothing to do with the Star coupler oddities as far as I'm concerned), OLM's are "smarter" than a repeater (poor little repeaters don't even get to have their own manual, OLM's and OBT's do). OLM's canfor example be used to built a ring (can't do a ringwith repeaters) to increase the network availability andthe ring consists of the same type of OLM (unlike Ethernet rings which require a dedicated Ring Manager switch). In the OLM ring casethough, Slot time andRetry limit need to be adjusted anda free address must exist below the HSA (another"oddity" that is often overlooked butthe "echo monitoring feature" reason behind it is rarely questioned). The Star coupler "oddity" though comes into playdue to"Segment monitoring" featureof the OLM's (which can be activated/deactivated via Dip switches and another feature that doesn't exist in repeaters). The only time it shouldbe deactivated for the RS485 channel is when a Star coupler setup isused. This is as good as it gets as far as my attempt of an explanation goes. Last but not least, I am confident that Siemens hasgood technical reasons for mentioningthe "watch out" list ifStar couplers are created/used, so I'd work on the principle on "why risk it if it can be avoided"(getting adetailed background technical explanation from the OLM development team on this matter would be great to have though, wouldn't it?). |
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