Driving the Data Bus

What do you know about the data bus? No, we're not talking about a Silicon Valley charter trip. Rather, data buses are the subsystems that move data between computers or components in a system, much like how your coach moves people from spot to spot — but much faster. Data buses are at the heart of motor coach multiplexing, and understanding how they work goes a long way in understanding how to use them.

A history
Electrical devices and systems have developed at a hyper-speed pace for the last 30 years or so. This progress has been in many parallel and not-so-parallel areas all at the same time, with different parties borrowing whatever technology, microchips and processors suit them at the moment. Computers and their natural offspring, digital communications, are the driving engines for most of this digital revolution. There is a lot of information on the Internet related to this subject, some of which can be confusing or conflicting, but the following should help you understand what data buses are and how they got on your bus in the first place.

Early in the computer game, it was sometimes necessary to have several digital devices (and/or computers) "talk" to each other. Engineers devised different methods to do this. Several of the standards used were adaptations of archaic legacy systems, and some were entirely new; out of these arose the data bus/data link systems that are now common.

To avoid confusion, the rest of this article focuses on automotive systems and will use the terms "data bus" and "data link" interchangeably. In addition, everything described below refers to a two-wire serial data communications link between computerized components.

Enter the SAE
Automobile manufacturers quickly found computers useful for managing vehicle engine, transmission and body systems. These computer systems were (and remain) largely proprietary, but the various committees in the Society of Automotive Engineers (SAE) started to work on common diagnostic data link standards and communication protocols for the mutual benefit of any manufacturers and vendors that might choose to use them.

In the middle of all this, the emissions enforcers in California (CARB) decided that emissions information could be gleaned from the vehicle's engine computer, so they invented the requirement for "OBD" (on board diagnostics). The problem was that everyone's diagnostics were done differently. Other states and the federal government also got involved in emissions testing. In order to simplify things, "OBD II" was then created, and SAE J1850 was implemented as part of this emissions monitoring effort. That was the first common non-proprietary data bus to be used on vehicles to any large extent. Today, this protocol is primarily used as a diagnostic and user interface system on cars and, to some extent, on large trucks and buses.

Truck and bus industry steps up
At about the same time, bus and truck manufacturers were beginning to realize that they would soon need all of the computerized components of the powertrain and braking systems to function together. This was a challenge in large part because there were so many proprietary protocols from different manufacturers. Common ground was needed. SAE to the rescue! The Truck and Bus committees of the SAE tasked themselves with creating and implementing new standards that would allow components from different vendors to communicate using a common Data Link/Data Bus protocol. The first of these, starting in about 1985, was the SAE J1587/J1708 (1587 and 1708 are different portions of the same protocol).

J1587 is used for drivetrain communications as well as diagnostics and user interface. The most current SAE Data Link protocol is called SAE J1939. Though it is similar to J1587, it is much faster and is suitable to being configured to tasks other than drivetrain management and diagnostics — even off-highway uses like agricultural and industrial equipment.

These days, many vehicles are being built with more than one data bus, and most motor coaches contain several separate data buses that may be linked together in various fashions. If linked together they will use a Gateway (electronic black box) to do the link-up.

Multiplex systems and their data buses
Without getting too technical, the multiplex modules all need to communicate with one another via a data bus. For instance, if the driver operates a switch connected to a multiplex module at the front junction box, the command is carried to a module at the rear junction box via a data link in which that module identifies a specially coded command and switches power on or off to a lamp in the back. The beauty of this system is that many multiplex modules are linked to a single two-wire data bus, thereby saving thousands of feet of wire and connections. Also, being computerized, they can be programmed to follow complicated operational sequences like lamp-test functions, etc.

What's important to know is that different multiplex systems use different data bus protocols. Those data buses do not necessarily follow any SAE standard. Some systems do use SAE protocols like J1939, and others do not. Multiplex systems in buses are proprietary to the maker of that particular multiplex system. Unlike engine diagnostic and communications data buses, they are self-contained, and there are no mandates as to what is or is not proper, as long as it works. While operation is generally simple from a user perspective, you can't assume they all work the same way.

Still want to know more? Watch your email for the March issue of Maintenance Matters in which we take a closer look at SAE standards and what they mean to your coach.

The FYI from MCI editorial staff values your feedback. Please e-mail any suggestions, comments, or ideas for future articles to

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