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Primary Connectors

Primary connectors are confusing. They are used differently for different types of families. There is information below (at least a start) that provides information on what the different primary connectors do in different situations.

It’s probably going to take me a bit to get all of the information about primary connectors documented and correct, but I’ve got to start somewhere…

Rule: A family can have only one primary connector per domain. Domains, as they are treated in Revit MEP today are Duct, Pipe, and Electrical (i.e. the three “high level” categories of connectors that you can have). This means that a single family can have as many as 3 primary connectors (one for each domain).

Rule: For each domain of connectors that a family contains there will always be a primary connector for the domain. If you have any duct connectors in your family one of them will be the primary connector. You can’t say that you don’t want to have a primary connector for duct unless you say that you don’t have any duct connectors.

Rule: Primary connectors have a “cross” or “x” drawn through them to indicate that they are somehow different than other connectors.

Air Terminals
There is a very specific behavior that occurs for air terminals related to the primary duct connector. When you select an air terminal you will see a parameter on the options bar that allows you to specify the flow of the air terminal. This parameter on the options bar doesn’t actually “relate” to a parameter on the air terminal, but relates to the flow parameter on the primary duct connector in the family. Changing the value of the flow parameter on the options bar actually does change any of the family parameters directly, but may change them indirectly if the flow parameter on the primary connector is mapped to a parameter on the family. There is no special behavior for primary pipe or primary electrical connectors on an Air Terminal family.

History: When we began development of Revit MEP we didn’t have connectors. Connectors weren’t something users really “think” about (connections might be, but connectors aren’t). Users did have a concept of a diffuser, and the diffuser had a concept of flow. So, we built that. If a duct got connected to the diffuser the flow that the user specified on the diffuser got pushed (or pulled) into the duct. Unfortunately as we continued to work it became less simple than to just have a flow on any given element. A Multi-Zone AHU for example will have a number of different flows. How do we control them all? To handle that we introduced connectors, but now we also introduced a problem with our previous model… Now that we can add connectors there is nothing that prevents the user from adding 63 of them. Do we put the flow into all of them in the air terminal case?... No, that doesn’t seem right, maybe if we had one that we somehow identified as the “special” connector. Yea, that might work. That’s why we put flow into the primary connector only (note: the primary connector wasn’t invented to solve this case, but it was convenient).

This specifically includes information about how primary connectors work on the following categories: Communication Devices, Data Devices, Electrical Fixtures, Fire Alarm Devices, Lighting Devices, Nurse Call Devcies, Security Devices, and Telephone Devices (there will probably be a few more categories added here before I'm done).
Behavior: There are 3 parameters on these categories labeled Panel, Circuit Number, and Electrical Data. Information from the primary electrical connector on these categories is used to fill out these parameters. Information from the non-primary connector can't be shown on the family. Primary Duct and Pipe connectors have no special behaviors when loaded into Device families.
History: When we started working on electrical we already had the concept of connectors from HVAC. We also had one category that everything lived in called Electrical Fixtures (the category existed in Revit Architecture (just called Revit at the time) before Revit MEP came along, so we just "borrowed" it). The initial workflow that we designed was the "user who shows wiring" workflow. In that workflow "most" of the tagging about panel and circuit information lives on the wires rather than the "devices" so it worked fine. As we started to look at the "user who doesn't show wires" workflow we quickly ran into a problem, as a "device" again, could have as many connectors as the user wanted to put on it. The "efficient" way of solving this problem was to solve it in the same way that we solved the Air Terminals problem, by having a single "primary" connector that "forwards" it circuit and other information on to the family so that it can be scheduled and tagged.
Includes both Duct Accessories and Pipe Accessories
Behavior: I can't find that there is any special behavior related to Duct Accessories and the primary connectors from any of the domains.
Electrical Equipment
As far as I can tell there are no special behaviors related to the primary connector when used on an Electrical Equipment family.
Generic Models
As far as I can tell there are no special behaviors related to the primary connector when used on an Electrical Equipment family.
Lighting Fixtures
Lighting fixtures behave very similar to the other electrical devices described above. The Panel, Circuit Number, and Electrical Data parameters are set based on the primary connector. One additional behavior is that the switch ID of the lighting fixture is also based on the primary connector on the lighting fixture. The switch ID's value is the switch that is assigned to the primary connector. The switch ID assigned to the non-primary connector is not accessible at the family level.
Mechanical Equipment
As far as I can tell there is no special behavior related to the primary connector for Mechanical Equipment today for any of the domains.
"Fittings" in general will cover both duct fittings and pipe fittings, as internally they aren't (for the most part) treated very differently. Fittings are one of the hardest things to document, because the behavior varies slightly based on the part type for the fitting.
  • Transition: There are some "assumptions" in the code about where the primary connector will be placed, so that the code can figure out how to correctly place the transition. There is an "un-discoverable" rule in fitting creation that requires that the connectors on a fitting "intersect" at the default intersection of the Center (Left / Right) and Center (Front / Back) reference planes. The code, in the case of a transition, also expects that the primary connector will be located at this intersection to work properly. With the primary connector set correctly you get the image on the left when you make a size change, and with it set incorrectly you get the image on the right.
TODO List:
Duct Fittings
Pipe Fittings
Plumbing Fixtures