This is a commonly asked question, and the short answer is that try to keep the expansion boards close to the main controller, and extend the wires to the valves. The wires to the valves can run as long as hundreds of feet; the wires from the expansion board to the main controller should be kept short because they carry logic-level signals and are prone to signal interferences. That being said, I’ve honestly never done scientific tests to see how far the expansion wires can go. Chances are they can probably still run pretty far (for example, you can make a long expansion cable yourself by modifying an Ethernet cable). The concern is that during bad weather such as thunderstorms this may become unreliable. The better solution is to use encoded signals, but that will require some significant change of the expansion board design.