Re: [PATCH v10 0/7] Add Fieldbus subsystem + support HMS Profinet card

From: Sven Van Asbroeck
Date: Wed Apr 17 2019 - 10:37:25 EST

On Tue, Apr 16, 2019 at 5:21 PM Enrico Weigelt, metux IT consult
<lkml@xxxxxxxxx> wrote:
> Yet another question: does each fieldbus_dev instance talk to exactly
> one plc process memory, or can there be many ?

I'm by no means a fieldbus expert, so I had a little chat with one of
the fieldbus
people in the company here.

AFAIK if multiple plcs connect to the same device, all of them 'see' the same
process memory for that device. Any mechanism to prevent these plcs from
racing with each other is implemented in the application software. Could
be handshaking, could be each plc dealing with a separate section of
the process memory.

So for a fieldbus *device* (which is what this subsystem deals with) there can
only be a single process memory. So we have a single devnode per device.

A fieldbus *controller* would have one process memory per device it connects to.
But this subsystem does not deal with controllers.

This is my understanding, but I'd appreciate your input if I have overlooked