MCI SERVICE INFORMATION

Multiplexing - How Does It Work?

"What is multiplexing? How does it work? Why is MCI switching to this type of electrical system? The old system worked fine, why make the change?" These are a sample of comments made when discussing the electrical system on the new MCI coaches, and not just by the owners of the vehicles. The industry as a whole is used to a "hard-wired" electrical system; following a wire from point A to point B when troubleshooting a problem. Understanding how a multiplexed electrical system works doesn't have to be overwhelming. The goal of this service tip is to discuss the multiplex (MUX) system using common, everyday language. As more is understood about the system operation, it will become less intimidating. Consider this brief synopsis your introduction to "MUX 101".

Beginning with unit 62032 on the E4500 and J4500, unit 51564 on the D4000/D4500, and unit 80413 on the G4500, MCI eliminated the stud board/ring terminal (hard-wire) style electrical system and created an electrical system comprised of several modules connected together and distributed throughout the coach. These modules perform the same functions as mechanical relays and circuit breakers, but with more flexibility and reliability. This also eliminates the need for flashers, timers, or other specialized components to make circuits operate.

Module and multiplexing basics are listed below. Point #1 is the most important, and it can clear up some confusion about system operation.

• Modules are connected to one another by a communication wire and they literally "talk" to one another electronically.
  
• Compare the communication amongst modules to the way people talk to one another on the telephone, similar to a conference call.
  
• Specific rules have been created so all modules speak the same language and talk in turn, creating a orderly conversation among the components (J1939 protocol).
• Each module is divided into two halves, inputs and outputs.
  
  • Inputs receive messages or signals from dash switches, pressure switches, etc.
  • Outputs supply power to energize lights, motors, etc on the vehicle.

• The modules have a microprocessor (computer) built into them.
• Each module is made of many circuits comprising the individual inputs and outputs.
• All modules are given instructions that assign jobs or tasks to the individual circuits.
  
  • These instructions are called programs.
  • The programs are written using special symbols called "Ladder Logic".
    
    • Compare these to arithmetic symbols.
• MUX modules used by MCI have diagnostic LED's on the face. These LED's are divided into 4 categories:
  
  • Input
  • Output
  • Power
  • Communication (network)

• The diagnostic LEDs are very beneficial in troubleshooting.

Components used in MUX systems are available from various vendors, but the basic principles are the same. The module pictured below was selected because the labeling on the exterior is very easy to identify and understand. See figure 1.

Figure 1

This module has 28 inputs and 20 outputs. In the upper RH corner are two additional LEDs, showing if the module is powered and if it is talking with the others in the system. (NET is the abbreviation for NETwork; PWR is the abbreviation for PoWeR.)

When troubleshooting a hard-wired coach, it is essential you have the proper wiring schematic. When troubleshooting a MUX coach, the schematic is needed but you also need the program logic (Ladder Logic). The ladder logic explains what is required for a specific function to operate.

Ladder logic symbols are arranged in rows with a column at each end, similar to a ladder, which is why these drawings are called Ladder Logic. See figure 2.

Figure 2

There are several types of symbols used to create ladder logic. In Figure 2, the first and third symbols represent two active inputs. The second symbol with the diagonal slash represents an inactive input. Whenever an input or output is active, the corresponding LED on the front of the module is lit.

The location of the input is listed above each symbol. The letter "I" represents input. The number following the I is the module number, and the number following the dash represents the input. The first symbol indicates that input 1 on module 3 needs to be active (input LED #1 is lit) as a condition to activate the output.

Outputs are represented by rectangles, with the location listed above, the same as the inputs. One difference is the location begins with an "O" (for output).

Ladder logic is read from input to output or left to right, as in this example. The ladder logic above reads: "On module 3, with inputs 1 and 13 active and input 21 inactive, output 11 of module 3 will be active." Reading the description below each symbol translates the operation of the circuit into English. The example in Figure 2 is for the step lights and for them to come on the following requirements must be in place:

• The master switch (ignition) is on.
• The entrance door closed sensor is inactive (in other words, the door is either opening or opened).
• The step light dash switch is on.

When you look at module three you should see inputs 1 and 13 lit, input 21 off, and output 11 on. You've just completed troubleshooting the step light circuit.

This has been a very brief lesson explaining the fundamentals of Multiplexing. If you have any questions, please call the MCI Customer Support Center at (800) 241-2947.