TechTips
Multiplexing Is Here To Stay
by Brainard Janicki Cost Drives The Practice Of Making One Wire Do Several Jobs
In the old days, some computers had 24 pins and some of the connector cavities were blank. Today, most computers have 60 to more than 100 pins and the only reason it has not grown even more is because of data bussing and multiplexing of circuits.Let's examine a common automotive accessory that uses multiplexing -- cruise control. The old method required four wires -- one for the on/off signal, one for set, one for resume and one for accelerate or coast. If the cruise control was located on the steering wheel, that also meant four brushes. On an airbag-equipped vehicle it meant adding four more wires to the clock-spring mechanism.
Today, with all the extra circuits, engineers had to find a way to eliminate some of the wiring. They had two compelling reasons to eliminate wiring:
1) There were no more empty cavities left in the computer and nobody wanted to add a second computer.
2) The other reason: cost. Well, actually from the manufacturer's point of view, cost was the only reason.
By multiplexing the circuits, the engineer could eliminate three of the four wires needed for a typical accessory like cruise control. That would allow them to build in more high-profit options without incurring much additional cost.
You can see why they're going with this approach. A side benefit with multiplexing is that the engineers have actually made the system easier to diagnose. We're sure this was pure accident on their part and we ask you to keep this secret to yourself. We sure wouldn't want engineering to think they had made our jobs easier.
However, the truth is that by attaching our voltmeter to a single wire, we can now check the entire operation of an accessory such as cruise control.
Here's how it works: One wire carries 5 volts from the computer to the cruise control switch. Inside the switch there is a series of resistors, all of them terminating at ground.
When the driver actuates the cruise control button, the 5-volt reference flows to ground through one or a pair of resistors. For example, when the cruise control switch is at rest, the 5 volts flow through a 22.6K ohm resistor to ground, causing only a slight voltage drop to 4.66 volts. The computer is programmed to see this as the system at rest. When the ON button is pressed, the 5 volts pass through both the 22.6K ohm resistor and a .68K ohm resistor. The resulting voltage on the signal wire will be 2.48 volts. The computer now knows that the switch is on and it will be ready for the next command.
Commands set/coast, as well as resume and accelerate, also have different resistance so that the computer can interpret their resulting voltages as separate commands.
The computer is programmed with a specific response based upon the voltage values it reads from the signal wire. The result? A single wire and four resistors take the place of four wires.
Obviously, cruise control requires other inputs such as vehicle speed, transmission gear information and brake switch information. This information is already in the computer and can be accessed through the computer's internal data bus. So, unlike the old days when separate cruise control wires ran to the brake switch, transmission and speedo cable, this newer method saves far more than just the three wires to the steering column. With this method, there is not a redundancy of wiring to each computer.
Multiplexing is also used in light switches, heater controls, electric mirrors, electric windows and electric seats, just to name a few.
Do you remember the bundle of wires that went through the driver's door post for windows, mirrors and locks? It can now be done with as few as two wires for each accessory. What a savings in cost and overall weight of the vehicle.
Tech Support Calls Carry Value
Customers need to know that no technician, no matter how good, can know everything necessary to fix every problem on every vehicle. No technician, whether dealer or independent, has the time to keep current on the 1,500,000 pages of technical literature.This cruise control article shows a good example of how simple accessories have turned into complicated multiplexed devices.
Technological advancements have changed the way every profession does business. Doctors, lawyers and professionals of all kinds routinely turn to specialists to help them solve tough problems. Those professionals charge for their expert consultations and serve as examples that such expert information is resellable.
Brainard Janicki has 11 years experience as a Chrysler quality control and lead driveability technician. He has worked in the Chrysler section of AutoLine for five years. Brainard holds both master and L-1 certification.
Each month, technicians from AutoLine Telediagnosis (http://www.autoshop-online.com) share valuable TechTips with our readers. As an ASA member, you're already signed up to use the AutoLine service at a discounted rate.
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AutoInc. Magazine ®, Vol. XLIV No. 9, September 1996