design philosophy (PDF)
The following pages describe two very distinct philosophies of
instrument system design. The centralized approach limits
flexibility by connecting sensors and displays to a central processing
hub, while the distributed philosophy emphasizes adaptability, creating
a more robust system.
One philosophy for
integrated instruments is a centralized design where
everything connects to the processor (except the displays, which are
shown here is not particularly large; (boatspeed, wind,
compass, GPS, loadcell, depth/temp and a laptop). But as the system
gets bigger, problems start cropping up.
is a maximum number of
and mix of input
types (analog, serial, pulse, etc.) that can be accommodated by the
processor, set at design time. Once you reach either
limit, you’re done, even if you haven’t used up all
For example: The designers of the WTP2 had this issue in mind. It has
30 connectors including 12 serial ports, 14 analog ports and 3 pulse
(i.e. boatspeed) ports. In order to accomodate them all, they had to
make the box really big; almost a foot square and 7" deep, weighing in
8 lb (not counting wires).
And you have to tell them in
what you plan to hook into it so they can custom-design the program to
them. And even then, they were blindsided by B&G's acquisition
by Simrad. They hadn't planned for the SimNet, which B&G is now
being forced to move to.
|The wiring is difficult to maintain and
You have to dismantle the processor in order to change or test the
cables, and when there are a lot of them, just touching something can
cause new problems.
The weight of the sensor and control
home-runs mounts up. The extra cables for the example centralized
system adds an extra 16
30 foot boat!
philosophy is to put everything on the same bus as the
displays, with small interfaces serving as connections for sensors, and
located at the most convenient point.
is the same system but designed with distributed architecture.
- The number and type of processor inputs
Each sensor is mated to a small interface nearby, and there is plenty
of room on the processor for the few cables that make sense there. The
processor box can be smaller and lighter. The input requirements for
the sensors are now independent of the design of the processor.
- The system is easy to maintain and
troubleshoot. Each interface can include local diagnostics
(e.g. status lights). And if you have a
problem with heading, you know where to go, and there are only two
wires to look at, with plenty of room to do so.
- You save weight by eliminating
control button cable