Use of RS485 on fieldbus devices

[PranavJ]

One of my customer asks me to use Power conditioner on fieldbus H1 segment with RS 485 capability. can some one suggest why is RS485 needed when we can get all the data via the H1 segment in the control system.

Pranav

[PranavJ]

Can you suggest me what is the use of RS485 connection on a fieldbus power conditioneer device.
:)

[Mike ONeill]

I think that there is only one power conditioner system that has RS485 connections (P+F) and those are from the diagnostics module only. If your client is requesting that capability, he seems to be pre-disposed to the P+F power conditioner solution.
The RS485 connection approach allows physical layer diagnostic data transmission to take place without loading the segment being monitored. However, there are also diagnostic product solutions that provide that diagnostic data on a separate H1 segment, again not loading the segments being monitored and having the additional benefit of not needing a separate communications infra-structure.

[tklatt]

Hello PranvJ,
Mike is right, but these tools (which transfer physical layer diagnostic data via H1) do have a reduced functionality and you have to spend a separate LAS (one channel of your H1 card). By using a simple Ethernet to RS485 converter you can transfer these data (e.g all diagnostic data coming from Power Conditioners of one cabinet) via Ethernet. On such a Ehernet network you can connect several of these converters together.

Best regards

Thomas Klatt

Manager International Sales Support PA/
Business Development Manager Fieldbus Systems
GAM/GPP

Pepperl+Fuchs GmbH
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[Mike ONeill]

By 'reduced functionality' I think you mean 'much lower price'. There is always a functionality vs cost argument - how many 4/20mA & HART loops had scopes fitted to them all the time, eh? The P+F unit is very clever and does some very nice things when it is necessary, but in reality, it isn't necessary very often or everywhere. Its very expense makes it very hard to justify for standard installations, but it is superb as a diagnostic tool for the less skilled (the scope part is good to look at/easy to use but I think its bandwidth is about 100kHz; a 'proper' scope would be much faster and can show a lot more, but it takes some practice to get the triggering right and to interpret what is being seen).

In practice, we tend to offer both on big bids; a standard H1 diagnostic module for alarming on 'gross' errors (low enough cost to have on every carrier covering 4 segments), and a separate mobile P+F widget for use during commissioning and when the standard diagnostic unit (or the internal DCS diagnostics which is often quite powerful itself) throws up some errors that merit more detailed examination.