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Instrument design philosophy (PDF)

Design Rationale

Realization of an Instrument system
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.

The Centralized Approach

One philosophy for integrated instruments is a centralized design where everything connects to the processor (except the displays, which are daisy-chained).

The system 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.
There is a maximum number of connections 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 the plugs.

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 at 8 lb (not counting wires).

And you have to tell them in advance what you plan to hook into it so they can custom-design the program to accept 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 a challenge to troubleshoot. 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 button cable home-runs mounts up. The extra cables for the example centralized system adds an extra 16 lb. to a 30 foot boat!
Distributed design

Another 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.

This is the same system but designed with distributed architecture.

Advantages:
  • The number and type of processor inputs are no longer limited. 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 sensor and control button cable home-runs.
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