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What is multiplex.... 

Multiplex is basically a computer network system on a bus.
It provides power to all the different systems and components through a network of cables (data bus) and modules.  The data bus is many times smaller than conventional wiring harnesses and therefore takes less wire and amperage to operate the electrical system.
The system has a main bus module or MBC.  The MBC is like the main computer where all the commands, or inputs, are recieved and come from outputs.  The system also contains several digital input and output controllers (DIO).  The DIO's in the system wait for the commands from the MBC.  Once a command is recieved it sends that command to the approptiate device or component. 
The key is that the MBC and DIO's can send and recieve multiple commands simultaneously.  These commands are sent across the network (data bus) extremely fast at 115,000 kilobytes per second.  This is all done without relays.   

Why Multiplex ?

Because complicated system requirements make relay systems impractical.  Relay systems are not nearly as reliable.  Less wire = less wire connections = less chance for poor connections, which causes many failures. 
With Multiplex, adding additional electrical components is much simpler and faster.  And the multiplex system is much easier to troubleshoot and diagnose than conventional relay based systems. 
Multiplex utilizes current computer technologies to simplify the electrical system, it utilizes information exchange through a single data bus, or cable.  There are very few moving parts, in some systems, no moving parts.  And it provides ease of maintenance with built-in intelligence. 
Multiplex will save lots of money, for a million different reasons. 

How does it work ?

It works with Module and Zone concepts, The Data Bus, Power Managment, and Inputs and Outputs.
MODULE is the network functional unit which contains the intelligent co-processor unit.  The vital communications data link to IO devices and inputs and outputs. 
ZONE is the locations of the modules in the bus.
Zone A contains the modules in the front of the coach.  There are two specific areas of zone A; the first consists of the two multiplex tell-tale light LED modules.
The other area of zone A is the modules under the curbside dash area.  Accesible throught the access panel in the step well. 
Zone B contains all the modules in the curb side junction box. 
The Data Bus is like a super highway where information travels throughout the network of modules at a high rate of speed.  It is  a cable which replaces large wiring harnesses, small gauge wires carry the information through the cable.  It only has 4 wires in a shielded cable.  THe data bus carries the flow of information throughout the multiplex system. 
The system will do a self test to confirm that all the proper modules are conected to the correct Data Cable and ensure complete communication integrity.  The self test will test both the main and second data bus cables.  If communication with the MBC (main bus controller) is lost, the COMM FAIL tell tale light will flash a code. 
Power Management The MBC is the only power source for the network.  It provides clean power to avoid data corruption from a common power source such as the battery, or from electrical interference.  A built in DC to DC converter in the MBC ensures a constant voltage supply of clean power throughout the network.  There is NO direct connection between the coach charging system and the multiplex modules. 
Inputs to the Main Bus Controller are supplied by the driver operated controls and various switches and sensors.  Basically, by turning on a switch, the driver has supplied an input to the MBC.  The MBC recieves Inputs as an ON or OFF wignal and determines which DIO Module it needs to call and give the message to. 
The MBC is the system supervisor, it dictates the work and tracks the progress.  The MBC uses the data cable to communicate with the module that controls the circuit needed.  Just like your home address, each module has it's own "address."  This address is used byt he MBC to contact the correct DIO module.  Each DIO module can control up to eight output circuits and receive 8 inputs.  The module then performs the operation that was requested, such as turn a light on or open a door.
Technically, inputs are ground circuits.  Once grounded, the circuit is active.  When open, the circuit is inactive.  Active and inactive circuits can be compared to a computer keyboard, with the shift key inactive, you get 1, 2, 3, etc.....with the shift key active you get !, @, #, etc. 
Multiplex can only understan ON - OFF signals, varialbe signals such as throttle position, temperatures, fuel levels have to be hard wired from the sender (sensors) to the reciever (modules).
Outputs... Only when the correct information is recieved will the multiplex perform an output. Some outputs only require one input (push horn button, horn blows) and some need multiple inputs (eg..the driver must have master switch on, WCL key switch on, fast idle on, trans in neutral, park brake applied, entry door open, for the wheelchair lift to work) 

Standard relay system operation Vs. multiplex operation

Relay system
Relays are used to allow low current draw circuits to control high current draw circuits, there by eliminating a lot of voltage drop. 
Energizing the relay coil changes the position of the relay contacts.  The load side voltage can be the same as, lower than, or higher than the coil control side.  There is no internal electrical connection between the coil and the load side of the relay. 
Multiplex system
12 or 24 volts will enter the module through the Data Bus cable.  Once an input signal is recieved and all the conditions are met, the corresponding Output (red LED) is illuminated.  The output signal uses a Solar Transistor to close the circuit to send the voltage to the appropriate component.  A 7.5 AMP fuse protects the circuit.  A yellow LED is used to indicate a load in the individual circuit.  (for those that don't know, LED stands for Light Emitting Diode).  If the Yellow LED is not illuminated, then one of two conditions exists......1. that circuit is spare and not being used, or 2. there is an open in that circuit as the load cannot find a ground.

The components, and how they work together

The main components are the MBC, the DIO, and the LED tell tale light cluster.
The G2A Main Bus Controller (G2A-MBC-32)
The MBC is the General in Command.  It has the special built in features such as a computer, which tells all the Seconds in command (DIO's) what to do.  It includes a PMS (power managment system) that provides 12 volts of clean and isolated power to the network.  It features 32 inputs, which can be source or ground reference.  The communication is at a speed of 115k baud rate.
Digital input and output controller (DIO)
The DIO module is a digital input and output module for ON/OFF state devices.  Internal feedback monitoring provides ability to determine if a load is active.  (yellow LED on)
Each I/O point is optically isolated and fused for protection against voltage spikes and short circuits.  DIO's are slave units, meaning they do what the MBC tells them to do. 
G2A-DIO-888 Module
The DIO module has several key areas.  The most important is the identification key.  This ID Key is how the MBC knows wehre in the network each DIO is located and what each DIO is supposed to do.  Three columns of LED's indicate Inputs, Outputs, and Fuses.  The small red LED nest to the three columns of LEDs is a status indicator.  Under normal conditions, this red LED should be flashing rapidly.  Each Output has a fuse for protection.  If a fuse is blown, the corresponding Output LED and Fuse LED will be illuminated. 
Dual Voltage Capability
DIO modules have split circuits to allow two different output circuit voltages (12 or 24 volts) from a single module.  Pins 1-6 recieve power on the red wire, Pins 7-8 recieve power on the yellow wire.  The power can be 12/12, 12/24, 24/12, or  24/24 volts.  This is dependent on the voltage needs of the circuit being controlled. 
Gray wire supplies power to the 8 inputs.  The white wire is ground.  Red and Yellow wires can be any combination of 12 or 24 volts.

Multiplexing Repair & Diagnostic Levels

There are three steps to diagnosing the cause of a failure.
Step One - Visual inspection for fault
Use the LED's on modules to diagnose a failed sircuit.  This will help locate 85% of all faults and is usually a part failure such as a bulb or a switch. 
Step Two - I/O control Test Kit
The test kit allows testing of modules and communication network.  It will help diagnose faults not found during visual inspections.
Step Three - Software Programming
This is testing and verification of module programming or programming of replacement modules.  Failed components can be returned to the manufacturer for repairs.
**There are no internal repairs allowed**
Using The LED Indicators
Checking a circuit is very easy.  Visually check to be sure the indicator lights are on or off when they are suppose to be.
Green LED is ON when an Input is active
Yellow LED is ON when and output is inactive but complete and OFF when an Output is active
Red LED is ON when and Output is active and OFF when an Output is inactive.
The MBC and DIO modules have green LED's to monitor all Inputs.  Each input has it's own address.  An illuminated green LED indicates the Input is active. 
**Reminder, all inputs to the I/O system are actually grounds.

LED's help to diagnose a faulty circuit.  If the Yellow LED is not lit on an inactive circuit, check the load circuit.  Yellow LED's can not get a ground if the load circuit is open  (bulb blown, solenoid burned out, broken wire, etc...)
If Yellow and Red LED's are both ON when the circuit is activated, check the fuse.  It's probably blown. 
If a module's Red LED's are all out, check the data link light adjacent to the data cable onthe side of the case.  Check the cable for loose connections or obvious damage.
If 2, 6, or 8 Yellow LED's are out, check the circuit breaker on the buss bar. 
The communications LED indicator is located adjacent to the 32 Input LED's on the MBC.  Normally this indicator LED will flash rapidly.  If it's slowly flashing, that indicates a problem with one or more of the modules in the network. 

The rest of the training manual provide specifics on understanding the diagrams and troubleshooting with them.  We do not have the ability to draw wiring diagrams on this server. 
This page is basically just an overview of the multiplex system.  The complete training manual can be obtained from any of our trainers throughout the country. 

The DogHouse......Your source for maintenance communication.

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