OpenSprinkler Forums Hardware Questions Wire Distance For Zone Expansion Board Re: Re: Wire Distance For Zone Expansion Board



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.