Design suggestion for a bus

Well because Devicenet exists, uses low cost wiring, is simple enough that you might make the nodes small enough so that you could have one per valve.

If you MUST have each node knowing its address without programming it must be plugged in to  a particular slot in the network (star connected) or be able to determine its position in  a daisy chained network.

Both methods are quite high risk - being very susceptible to mis-plugging in the field.

I'm sure that a position discovering daisy chain scheme could be devised but it's hard to imagine one that would be very reliable (all the obvious ways of doing it require nodes to actively process all messages that pass through them).

My hands on experience of building machines and control electronics says use star wiring, design a reliable hardware driver and replace the whole board if it fails.

If you can't get good enough reliability like that (and it wouldn't be good enough for some applications) then you are into a different realm alltogether and without very detailed information about the system I couldn't make any suggestions.

MK

What if you used something like a hall-effect sensor to see if your pneumatic actuator actually actuates?  Closing the loop a little farther out would cover more possible error conditions.

It sounds like were at an impass here

without a proper description of what solution wants to be realized, we are unable to probide any more than speculation as to what might work

It is obvious that if a specefic valve has a specefic function unique to the valve then there MUST be configuration, either by the position of the controller in a pre-defined bus (Star , Linked or otherwise) or by software or by jumpers. you can NOT have a parallel system where all boards are identical and with zero configuration or knowledge of position of each control and expect it to work... sorry not going to happen

the best thing right now is to provide a better description of the system operation, function and layout

Robert Peter Oakes

Take a look at token ring. That may  be a wonder.

I worked with Broken Ring many many tears ago

It woul dnot work in this scenario as while it is a sobust networking solution, it soes not discreminate between loation and function, only ensuring everyone gets to talk.

Network address and the like higher in ther ISO Layers get to decide function and identity from a user/ unique perspective

There is no impasse here

Each actuator must be automically aware of its position in the chain without user intervention. Configuration should happen automatically. That's why a standard bus does noto match my needs

Hi michael

Which information do you need?

How about this:

Any serial (or parallel) bus can be used.

• Each node has an input bus and an output bus.
• Each node has a watchdog chip that will connect input bus to output bus if the node processor fails to pulse the watchdog.
• The master sends an address byte and a command byte. If the receiving node address matches, then it acts on the command without passing the data to its output bus.
• If the node address does not match the received address, the data (address and command) are passed on to its output bus - the next node in the daisy chain.
• The master has a second input bus connected to the last output bus in the daisy chain of nodes - if the address makes it all the way back to the secondary master input, then that node address has failed.
• If the node does not have an assigned address (it just got installed) it permanently assigns itself the first address that arrives and acts on that command and all future commands to that address.

Doug

Hello Doug,

Your scheme has two issues that I think the Ambrogio wishes to avoid:

1) the signal is processed by each node so one dead node means that the chain fails - the master will know but can't address the remaining good nodes.

2) the master will know that a newly plugged node is node x of y nodes but this won't help if two are replaced at the same time (ie it isn't robust) .

You can mitigate issue 1) by duplicating the bus in a suitable fail safe way or by using relays to isolate a dead node and some kind of adequately reliable watchdog circuits to operate them. Without knowing the required SIL (check on WIKI for definition and brief discussion) it isn't possible to make detailed suggestions.

My personal take on it is that the problem, as described so far, is insoluble.

Using a human to correctly plug the parts in  a chain or a star is very unreliable (in an automotive production FMEA analysis we would have given it a probability of failure of 1% or worse) .

Any method of mitigating this issue involves identifying the actual valves.

You can't make  a reliable system  where the master operates the correct valve via a collection of interchangeable and identical nodes unless the master can identify individual valves .

One way you could do this is to embedded in each valve (or glue to the side of it) a reverse diode and a coding resistor, then the master could discover via the collection of drives and buses, which valve was where and operate it accordingly. (Can work with single driver board for all valves.)

Equally you could glue the driver board to the valve and program it with a unique ID (dil switch ?), and then use any star or daisy chain arrangement you like. The master can easily spot duplicate or missing nodes.

MK

It is tough to express a detailed solution in a few sentences, but what I was thinking is a failed node would be automatically bypassed (with a pass-through circuit such as analog switches) based on watchdog timeout.

If the system is re-powered when a node is replaced, each node would always end up with the same address as its place in the daisy chain, regardless of how many nodes get replaced.

Doug