Connecting incompatible drives & controls equipment – joining the dots

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Bringing together different pieces of equipment to make a coherent automated system can be a challenge. An increasingly frequent problem that many engineers face. 

Let’s say you have legacy equipment from differing drives & controls manufacturers, including ABB, Siemens, Control Techniques, Danfoss, Omron, Rockwell, Trend to name but a few.

Here I take a look at ways you can start to bring those differing assets together.

The problem

A production line can evolve over time with upgrades along the way, or new equipment comes in that needs to interface with existing assets.  

Equally, we often see equipment being re-allocated from another facility within a group which appears to have no common components with existing plant.

Sometimes, the old-fashioned way of using isolated digital IO (usually via volt-free relay contacts) works well enough, especially if there’s only a few points to consider (clear to send, line stop, etc).  Simple, easy to diagnose and effectively the lowest common denominator.  

The downsides come when it gets more complicated with the plethora of signals needed, if any analogue signals (line speed being the favourite) or tracking data are also needed.

Suddenly you are looking at signal isolators and, in some cases, whole cards of IO required to pass the required signals.  

What if it’s an obsolete system where IO card supply is limited or expensive?  

What if you’ve reached the IO or rack limit? 


  1. If it’s two different generations of the same manufacturers’ equipment, you may be able to use their generic fieldbus (e.g Profibus for Siemens) or communication platform as these remain a constant over several iterations of PLC.  At least here you should have the ability to pass large numbers of signals, both digital, analogue and complex data.
  2. Where you have equipment from different manufacturers, all may not be lost though.  There are some overlaps of “open” protocol communications; Modbus being one of the oldest and still in use today, for example.  It is also one of the few which works across 2 protocols – good old serial, as well as having an Ethernet version.
  3. More recently, you need to think more outside the box – or rather in another box in the form of a Gateway.  These devices almost act as a universal translator, bridging two seemingly incompatible fieldbus systems:  

Modbus RS485 to Profinet?  No problem.  

Profibus to CC-Link?  By all means.  

Set up a mapping table inside these clever little devices and they appear as a device both sides like to talk to.  

They’re also surprisingly cost effective – in some cases much less than a rack-based communication card.  

The only downside may be needing to modify the configuration on both sides – something that you may not have the luxury of – but this applies to most communications based solutions.

If you’d like more clarity on how to bring together seemingly incompatible drives & controls equipment, we’re here to help.

Blog post authored by: Phil HowardLead Systems Service Engineer, iconsys