Vehicle Communication Interface Module (VCIM)
The vehicle communication interface module (VCIM) is a 3-watt cellular device that allows the user to
communicate data and voice signals over the national cellular network. It is powered by a fused, battery
positive, voltage circuit connected through the vehicle wiring to the 3-button assembly and the radio, and
attached by means of coax cables to the cellular and navigation antennas. Ground for the module is
accomplished by means of dedicated circuits that are routed with body wiring systems to chassis ground points.
The module houses 2 modems, one to process global positioning system (GPS) data, and the other for cellular
information. Satellites orbiting earth are constantly transmitting signals of their current location, from which the
OnStar(R) system is able to pinpoint its own location. The navigation antenna receives these GPS signals and
provides the data to the VCIM to be processed. The VCIM communicates with the rest of the vehicle over the
class 2 serial data bus. The ignition state is determined by the VCIM through class 2 messaging. The module
also has the capability of commanding the horn, initiating door lock/unlock and operating the exterior lamps
using the class 2 serial data circuit. When an OnStar(R) keypress is made, a class 2 message is sent to the audio
system to mute all radio functions and transmit OnStar(R) originated audio. After the audio system is muted, the
OnStar(R) signals are transmitted to the audio system on the cellular telephone voice signal circuit, and returned
to the module on the cellular telephone voice low reference circuit. The cellular modem connects the OnStar(R)
system to the cellular carrier's communication system by interacting with the national cellular infrastructure.
The module sends and receives all cellular communications over the cellular antenna and cellular antenna coax.
OnStar(R) Button Assembly
The OnStar(R) button assembly may be part of the rearview mirror on some vehicles or a separate unit on
others. The button assembly is comprised of 3 buttons and a status LED. The buttons are defined as follows:
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The answer/end call button, which is black with a white dot allows the user to answer and end calls or
initiate the personal calling feature, if equipped.
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The blue OnStar(R) call center button, which displays the OnStar(R) logo, allows the user to connect to
the OnStar(R) call center.
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The emergency button, which will display either a red or white cross, sends a high priority emergency
call to the OnStar(R) call center when pressed.
The OnStar(R) button assembly receives 10 volts on the keypad supply voltage circuit. When pressed, each
button completes a circuit across a resister allowing a specific voltage to be returned to the vehicle
communication interface module (VCIM) on the keypad signal circuit. Depending upon the voltage range
returned, the VCIM is able to identify which button has been pressed.
The OnStar(R) status LED is located to the right of the emergency button on a mirror-mounted assembly and to
the left of the answer/end call button when the assembly is mounted on the dash or overhead console. The LED
is green when the system is ON and operating normally. When the status LED is green and flashing, it is an
indication that a call is in progress. When the LED is red, this indicates a system malfunction is present. In the
event there is a system malfunction and the OnStar(R) system is still able to make a call, the LED will flash red
during the call. The OnStar(R) LED is controlled by the VCIM over the keypad red LED signal circuit and the
keypad green LED signal circuit.
OnStar(R) Microphone
2004 Hummer H2
2004 ACCESSORIES & EQUIPMENT Cellular Communication - Hummer H2