Although Mid Speed GMLAN was originally slated to take care of entertainment and navigation systems, the engineering community came out with such sophisticated devices for these systems that they quickly surpassed the capabilities of the serial data network. For now though, on some of our more up-level models (think SAAB) fiber optics becomes the data line of choice. Fiber optics will probably see more utilization when they can be made more economically and durable. Fiber optic cable is lightweight, impervious to electromagnetic interference, and is able to handle data speeds of 25MB per second, so it is an excellent choice for intercommunication and control of today's automotive audio and telephone systems. Due to the high speed of the bus large files of data can be moved giving high quality real time audio and DVD reproduction. (MP3 players will soon be on board as well.
 | The SAAB 9-3 has a system referred to Media Oriented System Transfer (MOST) which has a baud rate of - are you ready for this - 25 Megabits per second! |
| Cables are 2 to 3 millimeters thick and are orange in color. The transparent inner core of the cable will be about 1millimetr thick. |
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Modules on the MOST network are arranged in a RING pattern with the radio
control head as the gateway to other data lines in the vehicle and is the master
of the bus.
The components must be "awakened" from a powered-down sleep mode and sequentially brought on-line. The system sequencing begins with an ignition-on, or radio-on, electrical wake-up pulse from the radio ("head unit"). This electrical pulse is send to all the components at once in a star configuration. At the same time, the head unit issues a series of "ring start" optical commands.
This sequencing continues as follows:
| (1) All the components "wake up." |
| (2) The CD changer receives a "ring start" command from the head unit. |
| (3) The CD changer passes the ring start command and the message "changer is here" to the telephone. |
| (4) The telephone passes the ring start command and the message "changer is here" with its own "telephone is here" message to the voice activation module. |
| (5) The Voice Activation Module passes the ring start command and the messages "changer is here" and "telephone is here" along with its own "Voice Activation Module is here" message back to the head unit. |
| (6) When the head unit recognizes the valid return of the "ring start" command and the "I'm here" component messages, a synchronous data ring lock is established. A continuous red glow observed in any of the optical connector cavities is the easiest way to identify that a ring lock has been established. |
When handling and replacing the fiber optic cable, the minimum bend radius is 25mm to insure the concentrated light reflection within the cable does not break down the internal fluorinated polymer reflective cladding. This can be the result of excessive vibrations (which makes it difficult for applications outside the passenger cabin). High heat will also cause problems, anything higher than 85 degrees Celsius will cause breakdowns in communication. The laser used in the MOST fiber optic system is known as a Class 1 laser which means that it has no known radiation effects. However you ca damage your eyes if you unplug a cable and look directly into the end at the flashing light. It is recommended that you hold a white sheet of paper to the end of the cable and see i light reflects off it.
Mik Stubing - Lead Instructor GMTCNY
