Server and communications group
Figure 2 is a larger view of what comprises the server and communications group. The server itself provides computing and storage; the GPS receiver, cell phone, and wireless LAN provide communications; and audio/video content stored on the disk provides material for later playback.
FIGURE 2: Server and communications group
You have two basic options for how you implement the server:
Build it in — You can package a small motherboard and the necessary peripherals into a rugged case and mount it permanently in your car. Doing so lets you put it out of the way, perhaps in the trunk or under a seat, and ensures that the power and wiring are both out of the way and perfectly integrated with your vehicle.
Build in interfaces and use a laptop — Alternatively, you can exploit the fact that laptop PCs are small and rugged, and that they already incorporate a keyboard, display, and speakers. You’ll likely want to keep the laptop in the passenger compartment with you, since otherwise you’ll lose the ability to use those peripherals, but keeping it in the passenger compartment requires that a lot of cabling connect to the laptop.
A laptop goes with you and brings its own battery power, but a Car PC server specialized for the purpose faces problems different from a server under a desk or in a rack:
Shock and vibration — The bumps and vibration you feel when driving, filtered by the springs and padding in the car seats, are significantly less than what a PC chassis sees when rigidly mounted to the car.
Limited space — The usual desktop tower or mini-tower won’t fit under the seat in most cars, and would use up far too much room if mounted in the trunk. The configuration of what’s in the server is less likely to change versus what’s at your desk, though, so you can make some compromises to reduce the package size. (Of course, if you really are installing the system in a motor home, a desktop tower would have some charm, because you’d have the space to build in an entire home entertainment capability in addition to the Car PC functionality.)
Temperature extremes and limited cooling — Cars must operate in temperature ranges from well below 0 to 120°F and above, and must survive temperature extremes beyond those limits. The passenger compartment will (eventually) come to within more benign ranges when the car is in operation, but there’s no cooling or heating while the car is off. If the PC is to be useful on demand, then it must survive and operate in any temperatures it encounters without fail and without damage.
Bad power — side from vehicle power being 12 VDC, it almost defies belief how bad the power is in a car. There’s noise from the ignition, a wide variation in the supply voltage, and loss of power only slightly more often than we get from our local regulated utility.
Figure 3 show the Opus Solutions chassis, specifically designed to host a car PC. It holds a Mini-ITX form factor motherboard with a two-slot PCI riser card to keep the size down, and incorporates a power supply designed to bridge between 12 VDC automotive ower and the computing electronics. Brackets on the bottom of the chassis let you anchor it securely to the vehicle body, while the compact, well-braced mechanical construction minimizes low-frequency mechanical resonances and improves durability.
FIGURE 3: OPUS Solutions Car PC chassis
You could build a shock tray to isolate the PC from those mechanical loads; this chassis is simply sturdy enough to withstand them.We don’t believe you have to worry about damag ng a securely mounted disk drive — for example, Seagate specifies its Barracuda 7200.7 series drives (Ultra ATA/100 interface, up to 120 GB capacity) for 63 Gs operating shock over 2 ms and 350 Gs non-operating shock over 2 ms.We suggest mounting the drive flat, not vertical, to minimize positioning errors caused by jolts to the head positioners.
The limited expansion possible with a small chassis and a Mini-ITX motherboard requires you to design the interfaces to each of the functions in your Car PC before choosing the specific components, because the server group must support the kind and number of interfaces required to connect to each subsystem in the car. If a specific interface your car subsystems will require isn’t supported by the Car PC, you’ll have to add some electronics to make up the shortfall. For example:
USB — The components we suggest use powered USB ports for the GPS receiver and cell phone Internet access. If you have only two USB ports on the server, you’ll fill them with this equipment; if you have fewer, you’ll need to wire in and mount a powered hub.
Serial ports — You’re likely to need a serial port for the touch panel interface (if it doesn’t use USB), two if you have two touch panel displays, and yet another serial port for the on-board diagnostics (OBD II) port. You’re not likely to find a small motherboard with two or three serial ports, so you’ll have to add them with a PCI card or USB adapter.
PCI — The choices for hooking up your wireless LAN, which you’ll use to load content on the server while parked in the garage or driveway, are PCI and USB.We suggest a PCI card so you have the data rate to support the faster IEEE 802.11g standard and don’t have to craft an external mounting for the unit. You have two PCI slots on a horizontal riser in the Opus chassis, so you could use one for the wireless LAN and still have a spare.