Byte order (Little/Big Endian) on ABB PLC PM564-ETH

5 posts in this topic

Hi Fellas,


Has anyone played with this ABB PLC and taken out data on its Ethernet port. The scene is like this..

This PLC communicates with a slave device on Ethernet and is also connected SCADA on the same network. (OPC on Iconics v9.32).  The slave device is a big endian. So we have set the ethernet port on the PLC to talk Big-Endian so that correct math can be done on the slave's data within the PLC. So far so good.

Now the OPC server for Iconics does not have the Byte-swap feature. When we connect PLC to Iconics, it is displayed as swapped! So the question begs.. When a ethernet port is connected to an f Big endian slave and a little-endian Scada at the same time, how does one proceed?


Thanks a million for looking in and will appreciate any help/advice


Raj S. Iyer




Edited by rajsiyer

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I am familiar with the concept of floating point/real byte/word order formatting that is sometimes termed big-endian or little endian which is very often encountered when dealing with  Modbus word/data formats.  And Modbus is a master slave protocol, where field devices are frequently a 'slave'.   But you do not mention Modbus as a comm protocol.  

I'm unaware of raw Ethernet formatting as big endian or little endian, so when you say that you "set the Ethernet port on the PLC to talk Big-Endian" I'm lost.   Since Ethernet is the transport layer, it really shouldn't care about data format, big or little endian.   Ethernet just carries bits back and forth.  It doesn't know or care what those bits mean or how they're interpreted.  The interpretation of what the bits mean is not Ethernet's job, the interpretation of the data bits is some application layer's job/task. 

So I'm guessing  that there's some comm protocol (Modbus or some other) moving data (slave to/from PLC) across Ethernet, the transport layer that involves the OPC server.   And that when you "set the ethernet port to Big Endian", you are really configuring the OPC driver's 'channel' to interpret data on that channel as Big Endian.   The data for that 'channel' happens to be on an Ethernet port, but the Ethernet port itself is neither big nor little endian, it's just Ethernet.

I could be wrong, but my experience with Modbus over ethernet where the Ethernet port also handles other protocols (FTP, HTTP, UDP) simultaneously confirms this.  

I know that Kepware's OPC server allows one to configure the interpretation of the data format by changing the word/byte order.   You have to know where to look for it though.

Iconics is a long-time player in the OPC server world.   In order to be competitive, I have to believe that Iconics also has that functionality, because that functionality is needed to handle the different formats encountered with Modbus.  

So I have to ask, where does the assertion "Iconics does not have the Byte-swap feature" come from? 




Edited by DanW

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Thanks for looking in Dan. You are right in every bit of what you say. However till date, I haven't found the byte order selection setting in the OPC server we're using for Iconics.. As you say, Indeed we're looking for it. Meanwhile we had to add a few rungs of code in the PLC to swap the problem variables that have to be shown on Iconics.


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I could not, in a couple of minutes, find a .pdf manual for the iconics OPC server.  They must have one, Kepware does:


See page 11 (pdf) for kepware's byte/word order choices.

Although there are four combinations of byte word order for 32 bit floating point values, I've only encountered two of the four.  It doesn't mean that some clueless vendor won't use one of the oddballs, but it does reduce the probability.

Do you have a URL for the driver manual for what you're using?

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