MULTIPLEXING

BY RUSS WALKER

Simply put, multiplexing enables the electronic and electrical components of an apparatus to communicate, or “talk,” to each other in a new and more efficient manner. Instead of using a multitude of copper wires between each switch, relay, and component on the apparatus, individual solid-state modules, or “nodes,” use microprocessors to communicate between each other to activate the selected circuit.

Conventional wiring

As shown in Figure 1, using conventional wiring, every switch in the system requires an individual copper wire from the switch to a relay to the device being activated. In some cases, for example, when the switch is in the cab and the device is in the rear of the truck, the quantity of wire and connections can be enormous, especially if several additional switches activate the same circuit. Every foot of wire and every connection is a potential problem area in which there may be a short circuit or a loose or corroded connection. These problems are the most common ones encountered in the apparatus electrical system.

Multiplexed system

In the multiplexed system in Figure 2, the input node is activated by the switch and sends a signal along the simple twisted pair of wires, called the “network data bus” to the master control module. The master control module then interprets the correct logic and continues the signal to the output node, which activates the device. In our example of the switch in the cab, numerous feet of wiring and potential problem areas can be eliminated with just this simple circuit. Because the nodes are solid-state and thermally protected, reliance on the old mechanical relays and breakers is greatly reduced. Multiple input and output “ports” on the nodes allow for several switches and devices to be controlled by the same node.

COMMUNICATION METHOD

The messages are sent from node to node over the CAN (controller area network) data bus and are simple 8-bit signals, just like the signals used in your computer at home. The signals transfer throughout the system at a rate of 250,000 bits per second, which allows plenty of room for all messages to get to and from the respective nodes. Additionally, using the communication network, the system can constantly monitor itself and any of the devices attached to it. Through the intelligence of the software, the multiplexed system can be “taught” to look for certain problems and conditions in the system and act on the information or indicate the condition to the operator on the system display. This communication tool will increase the operator’s ability to monitor and control activity on the apparatus.

MULTIPLEXED INTELLIGENCE

The advanced intelligence of today’s software takes over the job that logic circuits, wiring, and relays in the conventional wiring system have always done. Instead of hard-wiring each switch and device in the old bulky harness, the multiplexed system will be able to understand simple logic statements that will activate a device.

Typical warning light circuit, using conventional wiring

Figure 3 shows a typical warning light circuit, using conventional wiring. For the device to be activated by two different switches, additional mechanical circuitry is required. Traditional wiring harnesses require this activation to be transferred by wire, relays, and breakers.

Multiplexed system with nodes in place.

Multiplexing intelligence uses logic statements in place of mechanical circuitry to define its work (see Figure 4). When the right conditions are active in the system, the nodes will communicate with each other over the network and power the appropriate device. Traditional relays or logic circuits are no longer required.

TYPES OF NODES

Several types of nodes, or modules, are used throughout the apparatus to create the multiplexed network. Each of these modules has a specific job to perform.

• Universal System Manager. This master control module is the brain of the system. The original equipment manufacturer (OEM) loads the software, which contains all the logic statements, into this module. This module is also responsible for all load management functions including shedding and sequencing.
• Vocation Module. This module contains the information unique to the apparatus and allows the vehicle’s drivetrain to interface with the multiplex network. This module also controls all of the safety interlocks or flags that may be required for the apparatus.
• Input (Switch) Module. The ports on this node allow for connection of switches into the network. Depending on location and quantity of switches, there may be several of these modules on the apparatus-in the cab or at the operator’s station, for example.
• Power Distribution Module. When the network requires activation of a device, this module sends power to the appropriate location to drive the mechanism. Again, there may be several of these located throughout the apparatus, depending on where the devices are located. Solid-state circuitry reduces the reliance on the old mechanical relays and breakers.
• Data Logger. The data logger is much like the “Black Box” of the system. It keeps track of events in the network, such as circuit activation, faults, and other functions. This module is an integral part of the diagnostic function of the multiplexed network. Additionally, the Data Logger acts as a 12/24-hour clock.
• Information Display Module. This module is the onboard information and diagnostic center for the network system. It contains many pages of text regarding the configuration of the system and complete information on the wiring “diagram” of the apparatus. Diagnostics can be performed quickly and easily using many of the functions of the Display Module. Warning messages or information text may also be programmed to appear on the display screen.

• Modem. The modem allows for diagnostics and configuration of the apparatus from a remote service center or by the OEM using a standard telephone dial-up connection.
• Personal Computer or Palm OS. Although not a part of the multiplexing network, a PC uses a WindowsT-based interface to program and configure the electrical system in its entirety. Diagnostics and reporting are accessed from the system by a standard or laptop computer.

The Palm OS, one of the most recent tools developed, will enable the servicing technician to access the network for diagnostics.

• Other modules. Certain modules are available to perform specialized duties that may be unique to a specific apparatus. The Joystick Control Module and the Ramp Rate Control Module (for proportional valve control) are examples of task-specific modules that can work in conjunction with the multiplexed network.

DIAGNOSTIC CAPABILITIES

By far, one of the most powerful features of a multiplexed electrical system is the enhanced diagnostic function available to the technician. The system contains an as-configured wiring diagram in the onboard Display Module that keeps the schematic of the apparatus at close hand. Available are multiple reports that can help track all actions and associations in the system. The technician is able to enter the system by personal computer, the Palm OS, or even the Display Module to quickly diagnose the exact problem, often without even picking up a tool.

ADVANTAGES OF MULTIPLEXING VS. CONVENTIONAL WIRING

Multiplexing has the following advantages over conventional wiring:

• weight reduction;
• a more reliable electrical system;
• unmatched real-time, onboard diagnostic capabilities;
• fewer light-duty mechanical parts;
• advanced load management capabilities;
• easy reconfiguration;
• less apparatus downtime; and
• quicker and more accurate return of information to the operator.

RUSS WALKER has been a sales engineer with Class 1, Inc., in Ocala, Florida, since 1998 and has been involved in the utility motor vehicle industry since 1983. He presents electrical and electronic control training seminars for various organizations and fire mechanic schools and conferences. He is vice-president of the local Habitat for Humanity chapter, facilitating education and interaction between local fire departments and new Habitat homeowners.