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March 11, 2013 at 6:43 pm #22368
Do you know the maximum wire distance for the Zone Expansion Board from the main controller? I have plans to add backyard valves which are about 100 feet from where the controller will be, and the existing cable only contains 6 wires. It would be sweet not to have to run a new cable!March 11, 2013 at 10:58 pm #23408
In my opinion, the difficulty of running a Zone Expansion Board on a long cable is signal integrity of the serial clock. On a long cable, you will experience significant ringing. This ringing can cause double clocking on the shift registers. The most important part of such a serial link is the serial clock. If you have ringing on the data line or the register clock line, it is not too big a deal as it does not cause bad effects. Ringing on the serial clock line is a killer. The usual method of dealing with this is to use controlled impedance drivers and matching controlled impedance cables. The standard cable used in irrigation is not controlled impedance, so it makes it difficult to solve. There are various clock snubber networks you can try. However, you really need a good oscilloscope to optimize these.
Are you planning on sending the 24V AC for the same cable? If so, then be aware that you can get a voltage differential across the cable due to the large currents in the solenoids. Once you get it working, you will want to do extensive testings. It may appear to work with any one valve on, but then fail when multiples are attached or fail to shut off.March 11, 2013 at 11:51 pm #23409
I agree. The cable is sufficient to carry power over a long distance (as long as you use thick enough wire), but the issue is in the signal quality, especially if the mcu is sending out serial clock at a high speed. You can feel free to give it a try, but I suspect that it won’t be very reliable. Instead of wiring the expansion board 100 feet away, is it possible to place the expansion board close to the main controller, and extend the wires to those valves?March 12, 2013 at 2:14 am #23410
One drawback of expansion boards on any type of cable is that there is no data return. Therefore, there is really no data checking. If the output of the shift register were fed back to the Mega328, then the signal integrity of the cable could be verified. That way, the controller would instantly know that something failed in the cable or expansion board. This would eliminate zones being left on or failing to turn on due to equipment fails in the either the cabling, the logic power, or the shift register. Sure, the traics can still fail, but at least you would know that the correct data got to it remote location.March 12, 2013 at 2:41 pm #23411
Instead of wiring the expansion board 100 feet away, is it possible to place the expansion board close to the main controller, and extend the wires to those valves?
Yeah, that would be the alternative (and sounds like only) option. I already have an existing 6-conductor cable, and was hoping to avoid running another one. I figured 100 feet was too much, but I had to ask. 🙂
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