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The diagnostic connector allows test equipment such as the Handheld Diagnostic
Tool described in Chapter 6 to be connected to the J1708 data bus. The diagnostic
connector is usually located underneath the driver's side of the dashboard.
Note - The diagnostic connector is not part of the instrumentation and is men-
tioned for reference only.
The private data bus connects the Interface Module to the tachometer and the 2-inch
gauges. It carries data from the Interface Module to the tachometer and 2-inch
gauges. The private data bus consists of a pair of wires in the 5-wire cable assemblies.
It is not connected to the J1708 data bus.
Identically wired 5-wire cable assemblies make up the private data bus that connects
the tachometer and 2-inch gauges. These cable assemblies carry power, ground,
backlight power and private data bus signals from one gauge to the next. Their 5-pin
plugs are polarized and cannot be plugged in backwards. Identical pinouts allow ei-
ther end to be plugged into either of two identical sockets on the back of the gauges.
The plugs are held in place by a locking tab. Depressing the locking tab lets you re-
move the connector from the gauge.
Each 2-inch gauge consists of a printed circuit board and meter assembly, and a
scaleplate with programming tab. These parts are mounted together with a glass
faceplate and a sealing ring in a plastic, twist-apart housing. Figure 4-3 on page 4-4
shows how these parts are assembled.
The printed circuit board has a red warning LED, two LEDs for backlighting, a mi-
croprocessor that converts messages from the private data bus into LED and pointer
information, a pair of private data bus connectors, and small 5-pin contact assembly
that accepts the programming tab on the scaleplate.
The scaleplate's programming tab automatically configures a gauge to the function
shown on the scaleplate. Holes in the tab perform the actual programming by allow-
ing certain contact pins to make contact with the circuit board. A microprocessor on
each gauge circuit board converts the private data bus messages into meter drive sig-
nals. Figure 4-2 on page 4-4 shows the contact assembly and the placement of the tab.
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