Re: Ada for Automation

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Fri, 7 Nov 2014 03:44:38 -0800 (PST), slos wrote:

>>> This you cannot do with, let's say, Modbus.
>> 
>> Of course you can. There is no problem to query only data you needed over
>> ModBus. We do this all the time.
> No, that's not the same thing.
> With today field buses you are able to select what will be exchanged
> cyclically in a PDO (CANopen / EtherCAT / POWERLINK) or Assembly
> (Ethernet/IP).

EtherCAT does not do anything cyclically. The master must explicitly
initiate every single cycle. It is strictly client-server. All immense
configuration overhead and slave complexity could be spared if the master
simply queried slave's registers using slave ID and the data point ID.

> The device will prepare its answer accordingly.

Which is unnecessary overhead only complicating EtherCAT masters and
slaves. BTW, our master does not use FMMU and other useless stuff. Which is
why it is capable to run as many independent cycles as needed, all
simultaneously. It does not make sense querying temperatures and digital
inputs at 200µs, right?

> With Modbus, you can access of course the whole data set but you end up
> reading more data and filter out the one you don't need or make several
> requests to pick up only the data needed.

No. This is maybe an artefact of the libmodbus you are using, I don't know
it. But in ModBus (class 2) you can read/write any single register or any
sequence of adjacent registers.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de