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Thread: Dive CAN/ISCAN/CAN Bus - what's the fuss?

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    RBW Member sea_ledford is an unknown quantity at this point sea_ledford's Avatar
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    Dive CAN/ISCAN/CAN Bus - what's the fuss?

    My understanding of what CAN bus is and what it does is limited. I know Revo, ISC and Hollis all are using some form of it on their rebreathers. Does that mean it's really in the shearwater controller?

    Is it just a way that data is communicated between the computer, controller and solenoid, or is it more than that?

    What are the positives and negatives?

    It seems to be a fairly significant change in rebreather design, I'm just not knowledgeable enough to know if I'm for it, against it or agnostic.

    And searching for "can bus" is not remotely helpful

    Thanks,

    Chris

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    RBW Member Sambo is an unknown quantity at this point Sambo's Avatar
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    Re: Dive CAN/ISCAN/CAN Bus - what's the fuss?

    Chris I dive a revo with Can Bus.
    Can bus is basically a digital communication system. It is generally made up of 1 main controller and input/output modules along a bus (or set of wires) It is called a distributed system as you can distribute different modules around your car, factory, rebreather etc. The system only uses 5 wires in 1 cable to talk to any device connected to the bus so the wires can be thick and robust.

    For instance with the analog systems, between your oxygen cells and the computer on your wrist you need 1 wire for each oxygen cell plus a common, you also need wires to control your solenoid. So generally these wires are thin, prone to breaking from the constant movement of your arm etc. The can bus computer only has 5 Thicker wires which are much less prone to breaking.

    With the analog system the long thin wires running from your o2 cells to your computer are also prone to interference. Since your o2 cells only put out mV it is easy for the cable to pick up stray signals from the solenoid firing etc which may alter the reading you see on your o2 signals at your handset.

    With the can bus system because you have modules distributed around the rebreather, the o2 board is directly beside the o2 sensors. Your fragile o2 sensor cables can now be made very short. Just long enough to connect the sensor to the o2 board (eg 50mm). From the o2 board the signal is converted to digital and sent over the can bus to the other boards (Solenoid board) and your computer. The digital signal either gets through to the computer and it displays a value for 02 or it does not get through and the computer would display a fault (eg if you broke or disconnected the cable) The digital signal is not prone to interferance like the analog so no chance of reading an altered 02 reading on your displays.

    Another benefit of the can bus is once the signal is sent out to all the control modules eg. setpoint of 1.2, if the computer drops off the system the o2 board will continue to communicate with the solenoid board and keep firing the setpoint and reading the o2 without the help of the computer and therefore keep your setpoint.

    Imagine the computer as the main command centre, if it drops offline (unplugged/broken) the bus modules can keep following the last command given and work together to keep operating.

    Another benefit (applys to revo, not sure about others) is if one of your distributed bus modules dies, unplug a 5 pin wet mateable plug, remove the module, plug in a replacement and your back in business.

    Future upgrades are easy, for instance if a new product is developed for the rebreather (HUD etc) you can just unplug the canbus and any point, slip a new module inline, update the computer software and the upgrades are installed. No soldering, cutting, wiring just plug and play.

    So Can Bus is a robust communication system which adds flexibility for future upgrades, ensures correct information is getting through with each message, and reduces wiring that is prone to damage. It has been around for many years in industry and is proven. Being new to rebreathers, Can bus rebreathers will have and some have had some initial teething problems, but due to its plug and play ability, problems are easily identified and fixed.

    Negatives
    At this stage higher cost in most cases

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    John H Hanzl johnnyh is an unknown quantity at this point johnnyh's Avatar
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    Dive CAN/ISCAN/CAN Bus - what's the fuss?

    +1. You just saved me a whole bunch of writing. Nicely said.




    John Hanzl

    Author, Out of Hell's Kitchen
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    John Hanzl

    Author, Out of Hell's Kitchen
    www.outofhellskitchen.com

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    RBW Member wallen is an unknown quantity at this point wallen's Avatar
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    Re: Dive CAN/ISCAN/CAN Bus - what's the fuss?

    Here is a Wiki article that, although pertaining to automotive CAN bus, gives you a basic idea of what the bus system is and how it works.

    CAN bus - Wikipedia, the free encyclopedia

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    Re: Dive CAN/ISCAN/CAN Bus - what's the fuss?

    Nicely put. There are some other negatives, however.

    Biggest one is the added complexity and risk of failure. Where before you had a piece of wire carrying an analogue signal that could be sampled by whatever was interested, you now have a lot of conversion, messaging and addressing protocol being handled in software with the inherent risks of adding software and bugs to any system.

    In an analogue system (looking at the inspo classic by way of example) you've got a battery box and solenoid in the head, two controller paddles and a buzzer. All the finicky electronics is in the paddles, mostly away from the hot/wet hostile environment of the head.

    In contrast, following the CAN architecture we now have the controllers plus an O2 board plus a solenoid control board added to this arrangement and they're sitting right in the most horrible environment for electronics I can think of.

    So you've added a bunch of points of failure. You've added complexity. You've added more software. You've added a stack of communications protocol which isn't bulletproof.

    In short, you've added risk to the system.

    On top of that, despite all promises made a long time ago the DiveCAN(tm) protocol - in particular the message addressing scheme - hasn't been released to public domain. Which means you have massive vendor lock-in and, as more and more peripherals (HUDs, etc) go digital you now have added risk of protocol mismatch between vendors.

    Don't get me wrong - I love the idea of using CAN and, when I finally get a round tuit and replace my classic electronics with a custom package it will be digital. But it isn't a silver bullet, there are downsides as well as up.

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    RBW Member Sambo is an unknown quantity at this point Sambo's Avatar
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    Re: Dive CAN/ISCAN/CAN Bus - what's the fuss?

    Captain I agree with added complexity but not necessarily added risk of failure. If you add a shearwater to a manual ccr you add complexity and you add another failure point as you have a new piece of equipment that can fail too, but that added equipment may also add safety in other ways. I think it's difficult to draw a line between where added complexity is beneficial or not beneficial. In many cases added complexity may reduce the risk of failure as risk of failure is made up of both the number of possible failure points combined with the likelihood of the failure occurring. For example the chance of breaking a one of 8 thin analog wires may be quite likely but the chance of a canbus module failing in a complex system very low. In this case the added complexity may have increased possible failures but reduced the chances of failure. I used to work in the automation industry and a failure could cost tens of thousands of dollars, but after extensive testing and proving you will find can bus in nearly all industrial environments. I used to install it on underground drill rigs where constant high humidity, water and rough handling destroyed most things. Can bus did extremely well.

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    RBW Member uwxplorer is on a distinguished road uwxplorer is on a distinguished road uwxplorer's Avatar
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    Re: Re: Dive CAN/ISCAN/CAN Bus - what's the fuss?

    Quote Originally Posted by Sambo  View Original Post
    Chris I dive a revo with Can Bus.


    Another benefit of the can bus is once the signal is sent out to all the control modules eg. setpoint of 1.2, if the computer drops off the system the o2 board will continue to communicate with the solenoid board and keep firing the setpoint and reading the o2 without the help of the computer and therefore keep your setpoint.
    I don't know that this capability is implemented yet, assuming it actually could and would be advisable. It's a bit like trusting your gas gauge to run your spark plugs in case the engine module died...
    Plus if your SW dies, the O2 cells are read but not by you, so would you trust a black box (actually two including the solenoid chip) firing up your solenoid without visual feedback on the pO2?
    Finally, if there were two masters for the solenoid chip, talk about potential conflicts!



    At least on the rEvo, if you have no alternate readout (e.g. Dream), you better bailout, won't you?

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    RBW Member Sambo is an unknown quantity at this point Sambo's Avatar
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    Re: Dive CAN/ISCAN/CAN Bus - what's the fuss?

    Quote Originally Posted by uwxplorer  View Original Post
    I don't know that this capability is implemented yet, assuming it actually could and would be advisable. It's a bit like trusting your gas gauge to run your spark plugs in case the engine module died...
    Plus if your SW dies, the O2 cells are read but not by you, so would you trust a black box (actually two including the solenoid chip) firing up your solenoid without visual feedback on the pO2?
    Finally, if there were two masters for the solenoid chip, talk about potential conflicts!



    At least on the rEvo, if you have no alternate readout (e.g. Dream), you better bailout, won't you?
    Here is a quote from Paul R from another thread on this forum
    "are you sure the unit was 'OFF' when you removed the controller? if not the unit tries to keep you alive, as it has PPO2 and can fire the solenoid, it does not need the handset"

    So it is implemented already. Of course like you say, if you have no back up ppo2 display (all canbus rEvos come with a backup ppo2 display) you need to bail out. Be assured there are not 2 masters! The handset is the master and the can bus modules are the slaves but after the master has given the command, the slaves can work together to achieve the goal of the master while he is away :)

    If you dont trust a black box (with a working ppo2 display) you better not be diving on any electronic unit as they all have a "Black Box"

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    Re: Dive CAN/ISCAN/CAN Bus - what's the fuss?

    A mate dives a new rEvo and it's a sexy bit of kit. We were talking about the setpoint controller and how it handled failure etc, attempting to maintain setpoint even though the controller had died based on the O2 reading messages on the bus.

    Great idea. Except for that top three or four metres after your handset died on a setpoint of 1.3. Cue superbuoyancy :)

    I get what you mean about adding points of failure being compensated for by adding risk management improvements in other places, and how the overall balance of things may be an improvement.

    Like I said, when I go to replace the electronics on my Classic, it will be with a similar arrangement: multiple autonomous units doing their own thing based on a CAN messaging infrastructure. I think it's a good way to go. But it's not perfect and it's important (as with everything) to identify and understand the bad points as well as the good.

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    RBW Member uwxplorer is on a distinguished road uwxplorer is on a distinguished road uwxplorer's Avatar
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    Re: Re: Dive CAN/ISCAN/CAN Bus - what's the fuss?

    Quote Originally Posted by Sambo  View Original Post
    Here is a quote from Paul R from another thread on this forum
    "are you sure the unit was 'OFF' when you removed the controller? if not the unit tries to keep you alive, as it has PPO2 and can fire the solenoid, it does not need the handset"

    So it is implemented already.
    I had no idea. Where is it written in the manual?
    My training is from approximately a year ago, but I don't remember having been told that...


    If you dont trust a black box (with a working ppo2 display) you better not be diving on any electronic unit as they all have a "Black Box"
    I think that it is by now a proven mantra: do not trust your electronics and doubt your displays!
    So I 'trust' the different manufacturers of parts and components in the rEvo to have done their best, but I do not 'trust' that the parts will always behave as intended.
    You know Dr. Murphy, right?
    This extends to non-electronic parts, BTW.
    And your buddy...
    (I can't get the buddy breathing scene of Sanctum out of my mind. Great lesson. I think I am going to choose the tiniest buddies I can from now on).

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