Mixing fieldbuses

[Dirk Diggler]

Here's a question for all of you folks.

In a new plant scenario, lets say an 800 point I/O count with about a 50-50 mix of analog and discretes. I think it's a no-brainer to go FF where applicable for the analog control/monitoring. The question is, the remaining 400 or so points where we need shut-off valves, motor controls etc. what do we do? I have heard the Tower of Babel argument regarding using another bus, lets say DeviceNet, to do this control and tend to agree. I would prefer to do this the old fashion way, with 120 VAC I/o (or 24 VDC if that is your thing) and 4-20mA for any drive control.

The problem is that the sales-force from the system vendor management wants to use, continue to tell us that "we can do it". Obviously they want to do it for some reason which they are not telling us. I say anything is doable, but is it a good idea?

Being located in the shop, I don't really have alot of pull in the decisions, but, can someone put together a few bullet points to counter this "sales pitch". I think I know a few of them, but I'm looking for the experts opinion as to why it is not a good idea.

Thanks.