CAN-bus in rebreathers

[floridakid]

I heard that there are a few rebreathers on the market already and some that are coming onto the market that are using CAN-bus to supposedly allow for different handsets and such to be integrated into the rebreather in a somewhat "plug and play" fashion.

I thought it was kind of interesting so I started looking into the CAN-bus technology and from what I understand CAN-bus requires some kind of host controller. My question is if you have a primary and secondary PPO2 display communicating via CAN-bus, what happens if the host controller suddenly stops working? Do both the primary and secondary go down or are the two PPO2 systems somehow isolated? Or am I just confused about the host controller thing?

[wedivebc]

I heard that there are a few rebreathers on the market already and some that are coming onto the market that are using CAN-bus to supposedly allow for different handsets and such to be integrated into the rebreather in a somewhat "plug and play" fashion.

I thought it was kind of interesting so I started looking into the CAN-bus technology and from what I understand CAN-bus requires some kind of host controller. My question is if you have a primary and secondary PPO2 display communicating via CAN-bus, what happens if the host controller suddenly stops working? Do both the primary and secondary go down or are the two PPO2 systems somehow isolated? Or am I just confused about the host controller thing?

The ISC Pathfinder uses CANBUS and has both a primary and secondary controller. The only common component between the two are the O2 cells.

[CFU]

I heard that there are a few rebreathers on the market already and some that are coming onto the market that are using CAN-bus to supposedly allow for different handsets and such to be integrated into the rebreather in a somewhat "plug and play" fashion.

The rEvo RMS system also does feature this, it has a CAN Bus system where one node the O2 sensor board reads the sensors, another node the solenoid board fires the solenoid, the shearwater is a node and I think the Bus Controller is inside the battery box to which all other nodes are connected. The O2 sensor board has a vacant generic connector which can be used for future applications.

I thought it was kind of interesting so I started looking into the CAN-bus technology and from what I understand CAN-bus requires some kind of host controller.

This is what I recall back from my time at university as well. But I am sure the guys who are currently involved with the DiveCAN can comment on this much better.

My question is if you have a primary and secondary PPO2 display communicating via CAN-bus, what happens if the host controller suddenly stops working? Do both the primary and secondary go down or are the two PPO2 systems somehow isolated? Or am I just confused about the host controller thing?

On the rEvo therefore you have a complete isolated analogue system with rEvo Dream display and the HUD. So even if one or more components on the CAN bus go down I still have a completely isolated display system. In rEvos case this can be "1 Dream - 1 cell" "1 Dream - 2 cells" "2 Dream - 2 cells" + you have the constant mass flow valve which is sweet because in case of electronics problems you can easily convert your rEvo in a fully working mCCR.

Cheers

Christian

[DwayneJ]

The rEvo uses Canbus to fire the solenoid, read three cells, and read temperature probes. The revo has an additional connection for future accessories. The Canbus cabling has redundancy built in.

Secondary PPO2 monitors such as Dreams, or second wired shearwater are analog so no direct interaction with the canbus. I have Cell 1-3 on Shearwater/Canbus, Cell 4 is analog on a rEvo Dream, and Cell 5 is analog on a hardwired Shearwater... No splitters are used on any of my cells. As above, loss of canbus and I can run my unit manual.

[CFU]

The Canbus cabling has redundancy built in.

I am curious what Paul might answer regarding this point. This would mean that the RMS runs 2 redundant bus systems with 2 independent host controllers and I cannot find anything about that anywhere. Now battery is redundant because the system can run either on the 9V or draw from the Shearwater battery. If one node, cell board, solenoid board, Shearwater... goes down not a problem the others will notice and still work i.e. measuring PPO2 and firing solenoid even though Shearwater is down. So the system is quite failure tollerant, but what happens if the controller (battery box) goes down? Does it really have redundant bus? Not that it is likely to happen or common but just what if. Also with my dreams CMVs and the manual add valve I can still end the dive on the loop.

[SimonK]

CAN bus does not require a 'master' controller, the clever thing about it is that message priorities and collision avoidance is all built into the hardware of each node. This hasn't stopped people inventing schemes that require a master node and all sorts of other unnecessary crap in attempts to 'improve' on the original idea.
Obviously each node on the bus requires some computing power and an interface to whatever equipment the node is supporting. If a node fails then data from that node is lost but everything else carries on quiet happily.

[CFU]

CAN bus does not require a 'master' controller, the clever thing about it is that message priorities and collision avoidance is all built into the hardware of each node. This hasn't stopped people inventing schemes that require a master node and all sorts of other unnecessary crap in attempts to 'improve' on the original idea.

this is not standard CAN bus technology then the definition is like follows:

"Host processor
The host processor decides what received messages mean and which messages it wants to transmit itself.
Sensors, actuators and control devices can be connected to the host processor.

CAN controller (hardware with a synchronous clock).
Receiving: the CAN controller stores received bits serially from the bus until an entire message is available, which can then be fetched by the host processor (usually after the CAN controller has triggered an interrupt).
Sending: the host processor stores its transmit messages to a CAN controller, which transmits the bits serially onto the bus.

Transceiver
Receiving: it adapts signal levels from the bus to levels that the CAN controller expects and has protective circuitry that protects the CAN controller.
Transmitting: it converts the transmit-bit signal received from the CAN controller into a signal that is sent onto the bus."

Cars i.e. use redundant bus for security relevant systems...
I am curious about the explanations though.

[SimonK]

That sounds like the description of a single node, it consists of a CAN controller which links the host processor to the data bus. The host processor is the item that is performing whatever function the node is there for, e.g. Reading cell voltages, operating the solenoid or running the handset.

Edit: these days the CAN controller and host processor are usually on the same piece of silicon.

[CFU]

it consists of a CAN controller which links the host processor to the data bus.

Thats the Transceiver... A controller collects informations forms it into a message, manages interrupts, handles priorities... still its what I recall from my studies back at University...

[SimonK]

Thats the Transceiver... A controller collects informations forms it into a message, manages interrupts, handles priorities... still its what I recall from my studies back at University...

Each node has a controller that does what you describe above, it takes /sends data from/to the host processor, message ID and up to 8 data bytes , and puts it on the bus in the correct form with the correct timing, manages collision avoidance etc. The transceiver physicaly waggles the volts on the bus and is little more than a glorified RS422 driver.

In an imaginary rebreather you could have four nodes, the handset which acts as the man machine interface displaying any relevant data, an O2 controller that reads the cells and drives the solenoid, a CO2 sensor and a HUD. Each node has a processing element and a CAN controller. Any one node could fail and the others would keep working.

CAN is a good choice for this sort of application but its not the only way, the inspo vision has been doing the same thing for years with RS485 or something similar.