Common pin on Zone Expansion

[Nick]

During the next hardware refresh, I would love to see a common pin added to the Zone Expansion module (unless someone can tell me how to pull one off the circuit board itself).

For my installation the Zone Expansion and OSPi are located the full distance of the control cable apart (about a foot). Having a common would mean that that valves plugged into the Zone Expansion are self contained. At the moment, I have a common wire that runs back to the OSPi for each Zone Expansion.

Thanks

[Ray]

I did think about this when designing the zone expansion board, but made the conscious decision of not adding COM wire to the expansion cable. There are a few reasons. One is that I assume the expansion boards are always placed close to the main controller (conceptually they should be considered as part of the main controller). As such, one COM port on the main controller is sufficient. Second, the COM wire carries 24VAC high voltage, while the other wires on the extension cable all carry low-voltage logic signals. In case someone accidentally plugged in the extension cable incorrectly (or made a custom extension cable but wired it incorrectly), having 24VAC wire mixed among the other wires increases the risk of damage. Also, adding the COM wire would require each expansion board to have a COM terminal block as well, which would require re-designing the expansion board enclosure.

If you have multiple expansion boards placed far apart from each other, you can run one COM wire from the main controller to the last expansion board, and the middle expansion boards can ‘tap’ into that COM wire as it runs by them. Just imagine the COM wire is an additional wire that runs along with the extension cable. Hope the ascii diagram below makes sense:

MC ———> EXP1 ———-> EXP2 ———-> EXP3
COM___________|______________|______________|

[Nick]

Thanks Ray. I’ve been thinking about this a little more…

I’d like to first say that I’m not trying to be argumentative. I’m just trying to spark a collaborative conversation that helps OpenSprinkler be the best it possibly can be.

• Because current is flowing through the valves, I think the voltage is going to be present on the common line as well as in the expansion board and on the expansion board interconnect. The voltage is AC so I’m not sure if that makes a difference
• The connector is already keyed so, personally, I wouldn’t be worried about not connecting things right
• I totally get why you wouldn’t want to redesign this (if it ain’t broke, don’t fix it)

[Shane]

I didn’t even think to look at this before purchasing. I plan to locate the expansion boards in different buildings, ie: main controller in garage, expansion board in greenhouse, etc. I’ll just run an additional wire for common when I build and run the custom cable for the expansion slots.

[Ray]

Another option, if you don’t want to run long extension cable, is to get a second main controller, and use the remote controller feature (as supported in firmware 2.1.6) to have the first controller manage the second controller. This is more expensive than just getting an expansion board, but considering the cost it saves in running a long custom cable, the net cost may be actually cheaper.

[Author]

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