Designing an RV with RV-C.

If you are an RV manufacturer wondering how to get started with RV-C, you've come to the right place.

Why Multiplexing?

Multiplexing is a fancy word for a simple idea. It merely refers to providing a “party line” for different devices to talk on. A typical RV contains many electronic products – electronic controls run the generator, inverter, refrigerator, air conditioning, and many other systems. And most of those products have some method of communicating. But each speaks a different language, to the benefit of no one.

Multiplexing defines a common language for every product, regardless of the manufacturer or function. A single pair of wires connects every element in the network, linking every product together like the phones in a party line. Every component can talk, and every component can hear the conversation.

Just as the telephone changed the world in so many ways, multiplexing promises to revolutionize the RV industry in many important ways. Other industries have already embraced it – most notably the automotive manufacturers who use multiplexing in every car and truck built today. And while there are many reasons the RV industry is embracing the technology, three key factors are driving it forward.

Universal Diagnostics

If you take your car into the shop today, the mechanic's first action is to plug in a computer into its diagnostic port. That port is an example of diagnostics using multiplexing. It will soon be possible for RV technicians to access a similar port in the service bay of a motorhome, plug in a cable from their laptop computer, and within seconds be reading diagnostic information from the major systems in the coach. Just like their automotive counterparts, they will be able to perform tests, configure components, collect data, and print reports. A single inexpensive tool will work for every component, regardless of the manufacturer.

Service cost and quality is widely acknowledged as an important concern in the RV industry. By providing the same type of service diagnostics as the automotive industry, multiplexing will improve diagnostic speed and accuracy, reduce warranty costs, and improve the customer experience.

Embedded Intelligence

A common feature in today's coach is a safety interlock that prevents the coach from being driven when a slide room is extended. Surely a sensible feature, which brings to mind the question: why doesn't the coach have a similar interlock for the shore cord, bay doors, or satellite dish? The answer is simple: while putting in one safety interlock is fairly easy, putting in several is much harder – not just to install, but to troubleshoot and fix if a problem occurs. But multiplexing changes that dramatically. With multiplexing, adding additional interlocks becomes trivial.

And many other types of “embedded intelligence” become possible. Products can change the way they function based on the available power sources. Climate control systems can interact intelligently. Features that were once impossible become a simple matter of programming.

Design (and User) Convenience

Multiplexing makes many things possible. With it, you can have multiple controls for the same component. Or control multiple components from a single panel. Product controls can be customized for the specific application. The scope for the RV designer to differentiate their product, refine the controls, and improve the customer experience is expanded exponentially.

Of course, the most powerful reason the RV industry is aggressively adopting this technology is simple: The future demands it. Every similar industry – from automotive to farm equipment, aviation to marine – has already moved to multiplexing. It is becoming a basic customer expectation. Just as no computer beyond the most basic is sold today without a network card, the time shall come when no RV over a certain price point wouldn't be similarly connected. The question is not “Whether”, but “When”.

Why RV-C?

f Multiplexing means allowing electronic devices to talk on the same wire, using the same language, then RV-C is simply the description of the wire and the language. There are multiplexing protocols defined for cars, tractors, factories, and aircraft. RV-C is simply a protocol designed specifically for RVs, and supported by major RV manufacturers and component vendors.

RV-C is based on a protocol called “CAN”, or “Controller Area Network”. Developed by Bosch GmbH in the 1980s primarily for the automotive industry, CAN has become the basis for protocols used in all kinds of vehicles. Virtually all cars built in Europe, and a great many built in America, include a CAN network that connects the engine, transmission, anti-lock brakes, and instrument panel.

CAN is the steel girders from which RV-C is built. The rest of the structure is the work of the RVIA Technical Subcommittee for Multiplexing Electronics. Formed by the RVIA in 2001, the committee membership includes representatives of over two dozen leading RV builders and suppliers. The committee began its work by selecting CAN as the basis for RV-C, favoring the technology for its reliability, low cost, and wide availability of chips and tools. Since then, the committee has labored for two years to define the rest of the “language”.

The result of their work is a document available to the public through the RVIA. The essential elements of the work are complete, but just as the English language continually adds new words, RV-C will continually expand to include novel devices and new technologies. Ultimately, the process of maintaining and expanding the language will become the job of ANSI, the American National Standards Institute, with the close participation of the RVIA and its members.

There are alternatives to RV-C. RV manufacturers today can choose from several proprietary multiplexing systems, or they can build their own multiplexing systems by customizing components themselves. But RV-C is a far more effective way to get the best advantages of multiplexing.

Alternative systems cannot provide the full benefits of multiplexing. Although they typically can control coach systems, they cannot interface with the existing electronics and provide meaningful diagnostics. The monitoring options available are usually far less than with RV-C. Service and diagnostic procedures are not standardized. And the total system costs are rarely competitive.

But perhaps the most troubling problem with alternative systems is the inevitable vendor lock-in. The choice of a proprietary system vendor is a difficult choice to change. And the network limits the manufacturer's choices for other components – generators, inverters, tank sensors, et cetera – or force them to sacrifice functionality. The RV builder is doubly troubled.

But with RV-C, there is no network vendor – the network itself is in the public domain – and the manufacturer may choose components from any vendor participating in the RV-C effort. Already a large number of major vendors have announced products or otherwise indicated their intention of supporting RV-C in their products. And many others are simply waiting for the market to develop. Since RV-C is fairly inexpensive to implement and specification is publicly available, RV manufacturers can be sure to have plenty of choices for all kinds of components in the near future.

The Basics of Installation

There are just two steps to designing an RV-C compliant RV. First, the coach must be properly wired. And second, the components must be checked, and if necessary properly configured, for proper interoperation. The following section will consider those “software” issues. This section will describe the wiring required.

RV-C wiring is an example of a “bus topology”. The jargon simply means that RV-C uses one of the simplest possible methods of connecting multiple nodes. A “trunk” line is run through the RV, terminated on each end by a small (120 Ohm) resistor across the pair. Wherever necessary, “drops” are made using the same kind of cable to each node. The cable itself is a twisted pair, and the drop meets the trunk in an simple tee. The “high” line is connected to the “high” line, and the “low” to the “low”.

The bus topology offers several advantages over alternatives like “star” and “daisy chain” systems. Unplugging any node has no effect on the rest of the network. Troubleshooting the wiring is easy with ordinary tools – the “high” line at each node should have continuity with the “high” line at every other node, and the same is true for the “low” line. The resistance between the high and low lines should read sixty ohms.

There are a few limitations on the length and type of wiring. The RV-C specification allows for a variety of wire types, but for a variety of reasons SilverLeaf recommends using Raychem 2019E0309, a 20 gage cable that will support a total network length of over 250 meters.

Regardless of the cable chosen, the drops are limited to no more than six feet in length. Thus the trunk must be routed fairly near every RV-C component. Putting two components on the same drop is also prohibited.

There is no standard connector for RV-C components. However, one item that is standardized is the diagnostic port. This is a drop to a connector located in or near the service bay of the RV. It is used by service technicians, who will plug their laptop service tools into it to get diagnostic and configuration information. The plug is a nine-pin connector from the Amp Circular Plastic Connector Series. (The typical part number is 206705-1, but variants are available to allow different mounting methods.) A dust cap should be placed on this connector when not in use.

Selecting RV-C Components

Although RV-C allows products from any vendor to be inter-operable, an RV-C network is not “plug-and-play”. The RV manufacturer has to take certain steps in the design process to ensure that all the components in the network properly work together. In the manufacturing process, the builder may also need to do some special configuration as well.

RV-C defines a standard way for products to talk, but it doesn't guarantee that every product says what needs to be said! For example, it defines a way for the generator to broadcast the amperage that it is generating, but it does not mandate that the generator broadcast that information. After all, the generator may not have any means of measuring that amperage. If another component is depending on that piece of information, it may not function properly.

Thus every component vendor is encouraged to provide as part of their product documentation an “RV-C Application Guide”. The guide documents the information that the product generates, the data it requires from other nodes, and other key facts that might affect how it works with other products. This guide may be just a single page, or it may be a small book, but it is crucial to the design process.

An Application Guide will refer to “PGNs”, or Parameter Group Numbers. A PGN is simply a list of related data items appropriate to the product. A product may have several PGNs associated with it. The PGN structure is determined by the RV-C specification, but the component does not generally have to support every PGN defined for the product, or every data item in a PGN. So although the RV-C specification may define four PGNs and forty data items for, say, a generator, it is quite possible for a particular generator to support just one data item from one PGN.

The designer's first job is to collect the Application Guides for every component intended for the network. For every datum required by a component, the designer must verify that another node is supplying it. The designer must also consider when the datum is needed – different components may have different power sources. For example, if the slide room electronics are only powered up when the ignition key is on, and another component needs the slide room status, there may be a problem.

The designer must also look for pitfalls such as having two sources of the same information. In such cases the designer has to determine which source is the most relevant, and perhaps adjust the configuration of various components to use the proper source. It is possible that two components may be completely incompatible, although this would be a sign of poor design by one or both vendors.

The designer must also look out for problems with multiple items controlling a single device. For example, suppose a radiant tile heat system is installed that can be turned on and off by the main thermostat, or manually by a multifunction touchpad. The problem comes when, say, the manual switch turns it off – and then the thermostat turns it back on! There are several ways of dealing with this type of problem, but it is the designer's responsibility to ensure that some solution is implemented. For major components such as the generator these methods are an explicit part of the RV-C definition.

One final pitfall is easily detected. Every item on an RV-C network has to have a “Source Address” - an assigned number from 0 to 255. Some products may support “dynamic addressing” - that is, they find an unused address when they first power up. But some products use “static addressing” - their address is “hard coded” into their firmware. No two products with the same static address can be installed on the same network. Addresses are assigned by the product type, so this should be a very rare problem.

The Control Panel

RVs equipped with an RV-C network are likely to feature at least one multi-purpose control panel. This panel – and there may be more than one – will provide the RV owner with a central place to monitor and control all the elements of the system. Thus it may combine the tank level monitor, generator control, inverter panel, and perhaps much more. There may be more than one such panel – perhaps a smaller control in the bedroom, perhaps a combination house and engine monitor in the dash.

Although a simple panel with just a few functions may be available off-the-shelf, the main control panel for almost any RV needs to be customizable. At a bare minimum it must be configurable for the specific components in the network – for example, different generators may support somewhat different features. Ideally it should also be customizable to provide features that are coach-specific.

But there is no single “ideal form” for a control panel. Notepad computers offer a model for a high-end panel – a large color display, perhaps using a touchscreen, with rich graphics. Another concept being pursued by one RV manufacturer is integrating the panel into the ordinary televisions, controlling it with a small joystick. More builders are working with custom-built medium-sized graphic units, typically about 6” to 9” in size. These units are easier to install, and if well designed are simpler to operate.

But some manufacturers will surely opt for even smaller, cheaper units, which can offer the same raw functionality as the most expensive, albeit with less “flash” and a little more effort from the user. And while the main panel needs to have a display with enough flexibility to serve its many functions, there may be one or more special-purpose panels that could be as simple as a set of LEDs. Such simple devices may serve as a tank and power indicator in the service bay, or to control the generator from the bedroom.

Regardless of the type of display, the control panel is crucial to the success of the RV-C implementation. Customers will base their opinion of the entire network on the quality of this single component. Therefore RV designers should pay particular attention to this component, and select a vendor with a quality reputation and known responsiveness.

Troubleshooting with RV-C

Today, when a car enters a garage the mechanic's first action is to plug his service tool into the port under the dash. He immediately gets a diagnostic “scan” on the engine and other systems. When an RV-C enabled coach enters a service center, the procedure is exactly the same.

The diagnostic tool is most likely a laptop computer equipped with a special adapter. The software performs an initial “scan”, identifying every component on the network and getting an initial diagnostic condition report. The technician can then zoom in on any particular component and get more detailed information. In some cases they may be able to configure or control the device, and perhaps perform more detailed tests.

RV-C defines a diagnostic method, but the RVIA does not supply the actual tool. That tool shall be provided by private vendors – SilverLeaf Electronics has announced that they will have a tool available in 2004, and they will likely have some competition. Regardless of the tool vendor, the product should be able to provide diagnostics for products from any vendor.

The availability of electronic diagnostics opens a new avenue towards warranty and service cost savings. If the experience of the automotive industry is a guide, mis-diagnosis rates will be dramatically lowered. Time spent in troubleshooting will be slashed. And the potential exists to reduce paperwork as well. The diagnostic information could easily be printed in a standard form, or even e-mailed to the warranty center for processing. The total cost savings could amount to hundreds of dollars per RV.

The one type of failure that the standard tool can't solve is a wiring glitch. Fortunately such glitches are rare in the field, and the bus topology lends itself to easy troubleshooting using just an ohmmeter and basic wiring tools.