DH+ network questions

[Lordzorak 0]

I do not have any experience with the DH+ networks. I have a slc 5/04 that is connected to a Panelview 1000 standard. It is currently using the DH+ between the two and the plant wants to add a second Panelview 1000 standard. I know that you can daisy chain equipment along the DH+. My question is this: Is there a special connector that should be used or is it alright to merely stick both wires under the same screw, making sure there is a snug connection? The Panelviews will be the end of each side and will have the proper EOL resisters. This leads to the other question. Currently, they have a single cable between the Panelview and the 5/04, but no EOL resistors. Are EOL resistors required for a network with just two stops? I would still guess yes but the lack of them in this case made me wonder. Thanks in advance. Z

[Ken Moore 23]

DH+ networks are very tolerant of improper installation, until you exceed the tolerance. EOL resistors are specified, but you can get by without them if your cable run is not very long. I always install them in the field, but sometimes leave them off for bench testing. You can daisy chain without any problems, I believe the recommend minimum cable length is 10 feet or so.

[panic mode 246]

sorry, could not resist, sounds like something politician would say...

[Ken Moore 23]

He He Vote for me! I guess I should have said, you can get away with not doing a lot of the recommend installation practices, until one day, your network doesn't work. Is that better?

[paulengr 44]

DH+ uses balanced differential signalling. You can have up to several dozen nodes. There's no real serious limitation in terms of distance between nodes but there is a total line length limitation. I don't know exactly how the token is passed around (the exact protocol) and it seems that nobody else could easily discern it either because over the years, AB never had to face competition from unlicensed DH+ or RIO competition. If the cable is electrically "short" then no resistors are needed because the cable acts like an electrical short circuit. I'm not sure how long the cable needs to be before it is no longer electrically short for DH+ but a few feet shouldn't be a problem. Hence the reason that it is perfectly acceptable to do this over short runs such as on the bench or directly from the PLC to an HMI only a few feet away. As the cable gets longer you can no longer treat the cable as a short circuit. It's a transmission line. http://en.wikipedia.org/wiki/Transmission_line As a result, any time that you hit a discontinuity in the cable such as the end of the cable, this is going to cause a reflected wave which will cause destructive or constructive interference including effects like "ringing" which can screw up, distort, or destroy serial communications. The size and nature of this discontinuity matters. We're not talking microwave frequencies here so a little bump of solder, two wires twisted together, etc., really will not make a noticeable difference at all. Note that there is no particular problem with putting nodes "close" to each other at all, only total line length matters. Back in the days of standard block I/O, it was very common to see perhaps as many as 6 or 8 standard blocks mounted side-by-side on DIN rail with a very short (<12 inches) blue hose cable between them. If you connect the right sized resistor (match the impedance of the cable) across the ends of the cable, there will be no reflection at all because the wave will couple into the resistor and be harmlessly destroyed. Since the wavelength at which this happens is so long, you can be very cavalier though about your connections, splice cables, and generally do all kinds of things that would never, ever work with for instance Ethernet. I've seen a few RIO components (RIO and DH+ are electrically identical) which have 2 ports on them for wiring convenience. I've also seen a "double headed" connector for the same purpose. I don't know where to get these special connectors. But the vast majority of the connectors (the ones that come on AB gear) don't have any of this. You simply have to slip both wires into the connector and tighten it down.

[BobLfoot 301]

This post will sound like urban legend to some, but proves the robustness being hinted to. I once visited a customer where machine "A" made parts and sent them to machine "B". Machine "A" was using three SLC 5/04, Two PV's and a PLC 5 all connected DH+. A blue hose from each SLC and PV came to a terminal strip in the PLC 5 cabinet and the connections were made by STAR connection. with an EOL resistor on the PLC 5. Machine "B" was using two SLC 5/04, two PV's and a PLC 5 all connected DH+. A blue hose from each SLC and PV came to a terminal strip in the PLC 5 cabinet and the connections were made by STAR connection. with an EOL resistor on the PLC 5. DH Star "A" and DH Star "B' were seperated by approximately 500 feet and connected by solid #12 wires. And yes it worked until we wanted to a CLGX Bridge to the mix. Had to rewire the whole mess in daisy chain format. And replace the #12 with true "blue hose".

[Rod_Hackney 2]

If your going to have to be able to remove the node without disturbing the network, I would suggest the use of the Trunk/Drop topology.

[paulengr 44]

Naw. At least with AB and several competitor products, they all have plug-in terminal strips. Just pull the terminal strip out without disturbing the wiring any time you need to do this.

[Rod_Hackney 2]

Yes, are are correct if just unplugging a daisy chained network would not break continuity. It would be maintained by the connector. Suggest based on wiring being changed at node during installation / new installation.

[TimWilborne 108]

If you do chose to use drops, pay close attention to your cumulative drop length which is 100 ft on DH+

[Lordzorak 0]

paulengr: Would you have the part number for these terminal strips? Also, thanks for all who replied. I do appreciate it. Z

[paulengr 44]

1791R-RIOSCON This is a 6 position terminal block (looks like a Y shape, no need to overstuff terminals).