DH+ CONNECTION

[AKOR 0]

Our refinery has 8-pairs of Allen Bradley plc-5 systems with each pair controlling certain part of the plant. Each pair of processor is setup in redundant mode using Backup communication module(BCM) and 1771-ASB I/O adapter. Each plc pair is linked to YOKOGAWA Centum XL DCS via 1785-KE card for display and operation by the operators. My problem is how to connect all pairs to a common DH+ network so that I can view and monitor all processors through my laptop via a 1785-KE card. Presently I can only connect to the processors individually via serial port. Any information will be highly appreciated. A sketch of my network is attached for clarity. AKOR INNOCENT. PLC_NETWORK_SKETCH.doc

[paulengr 44]

I'm not 100% sure what the backup communication module does. I'm going to describe the standard PLC case. The BCM may explain what to do. If the backup is effectively "passive" in the pair, then you probably set them both to the same node address and everything will work. Otherwise...I've just never had to do that. If you've never done Universal Remote I/O, then this will be a bit confusing. If you have, it's very simple. Electrically both networks are identical. Use the Belden 3-conductor stuff, not the coaxial cable (unless you are a glutton for punishment). If you want to avoid conduit, both Belden and CommScope make the standard "blue hose" DH+ 3-conductor wire in an armored cable so that you can run connections with this stuff. First off, Ethernet is a much nicer, faster, and less cumbersome way to go. But, unless you bought Ethernet-capable processors, or you want to buy an aftermarket card (for your configuration, sticking a Digi One IAP in the middle of your serial runs would be a pretty simple way to do it), it can get very expensive compared to DH+ which is "free" relatively speaking. First, figure out what size termination resistors you need according to the AB manual (depends on length). Second, remember that everything will be in one long linear line electrically so figure out the best cable path to use. Second, configure each processor with a different node address using dip switches. You'll have to pull each processor out of the backplane to do this. Set it up from the software side as far as the DH+ stuff goes just like any other network standard. Same with the PC end although you do this entirely in software. Now begin with just your PC and one PLC to prove you know what you are doing. Wire each 3-pin connector up on each end in the way shown in the manual. If you have done universal remote I/O before, the wiring sequence is identical except blue and white are reversed. Electrically both network types are identical. Connect the termination resistors across the blue and white wires on the connectors. Connect the ground (bare) cable on ONE END ONLY. Do this for each physical cable segment of your network. If you ground at both ends, you will pick up nearby magnetic noise in the loop antenna you just formed via a ground loop. Which end you ground doesn't really matter as long as you do it on one end only although since a laptop/PC is usually ungrounded, that's the only place where I make an exception (don't connect it at a PC card). Go into RS-Linx and set everything up as far as adding a driver and a node for that particular PLC, just like you'd do for any other network. Verify that it's all working as expected. Now remove the connector from the PLC and pull out the termination resistor. Pull out the wires from the connector and twist the blue wires together for the next cable segment, and same with the whites. Again, ground on one end only (your choice). Put the termination resistor in the next PLC connector. Move on to plug in the next PLC and repeat the process. You do NOT have to have every connector connected at the time that you do this (you can leave a connector open). So eventually you'll be leaving everything connected. It's just that the termination resistors have to be present to get anything to work right. If they aren't, you get a reflection on the cable. It does sometimes work regardless but obviously not reliably. About the only thing odd you'll notice right away is that when you do uploads/downloads, and connect initially to each PLC, there will be a time period when you get some strange messages about node numbers simply because the .RSS files have the node addresses from using the serial ports and you are connecting via a different port. From a network perspective, DH+ is a token passing network. One PLC on the network holds the "token" and when it does, it transmits DF-1 packets encapsulated in the DH+ network protocol. When it is done, it sends some sort of "I'm done" signal. The next PLC in order (I believe by node address) then gets the token and can transmit. This pattern continues until every PLC has sent packets. Then the process repeats again with the lowest numbered node ID. All PLC's will be serviced in order and are gauranteed access as long as the network is intact. Electrically, it appears as far as I can tell as a sort of modulated RS-485 with some pretty high voltage signals (+/-12 volts?). Not your run-of-the-mill RS-485. You can't really get any more information than this because the whole thing is proprietary and carefully guarded by Allen Bradley.