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As until recently I have considered OPC DA and AE all I need, I never bothered to research what HDA offers exactly. Now, however, I have a principal project that would make use of data archiving in the PLC and HDA immediately came to my mind. It turn out that I can not find what exactly is HDA functionally. By itself, it's just a protocol to abstract historical data from presumably different sources, that's clear. It's not clear for, however, what the servers offer.

I imagine two possibilities. First, if there is an array/table/any standardised archiving in the PLC, the HDA server is an easy way to get all data in certain range, thus saving you the effort to make complex and lengthy logic in PLC and PC to extract the data. The second possibility is that the OPC HDA server just caches historical data on the PC where it is installed and makes it available to clients.

Obviously in the first variant, a HDA sever is would be much more useful, as it's normal to consider PLC much more reliable than PC, and thus data could be gathered there more reliably and then extracted asynchronously to the PC database.

Thanks in advance for your answers.

Regards,

Peter

Peter,

HDA stands for Historical Data Access and is a separate standard than OPC DA and A&E. Most PLCs and other real-time data generating devices would not have an HDA OPC server available because the real-time data would be feed using OPC DA and then stored into some sort of a process historian. The HDA standard uses the similar data structure for requests but includes start and end time - like the form (tagname, value, timestamp, quality, rangeSTART, rangeEND). Your point is valid that HDA Server for use with a PLC would be an interesting use, we have found that arrays, historical tags, etc. can be used to grab some form of history from the device and then transferred to the historian for long-term storage.

HDA is very useful for transferring quantities of process data (i.e. store and forward) from disparate locations via dedicated line, cell, radio, satellite. It's also valuable when there is a requirement for backup and/or data storage redundancy.

I hope this helps with the question. Feel free to contact me if you would like anything further.

Jeremy Eves
Regional Manager
MatrikonOPC
jeremy.eves at matrikonopc. com

Hi Peter,

The answer might change depending on which PLC and protocol you're using. Some protocols work well with HDA, some not so much. For others, such as the Matrikon OPC Server for DNP3 (http://matrikonopc.com/opc-drivers/345/index.aspx) the rolling buffer on the device is just used in case you miss any updates. Missed updates are bundled into an OPC DA 3 message, almost like a mini-HDA data transfer.

To answer your question as to what typical OPC HDA servers are used for; most offer up data from popular historians or expose an OPC interface from a relational database such as Matrikon's OPC Server for GDA (http://matrikonopc.com/opc-drivers/1757/index.aspx). More advanced HDA servers offer some aggregate reads such as average, variance, etc.

Again, the answer will depend on the protocol/PLC you're thinking of using.

Hi Peter,
s mentioned HDA stands for Historical Data Access and is one of the many OPC interface standards out there. From what I remember from my OPC certification training the majority of the world is DA, followed by a small amount of A&E then followed by an even smaller amount of HDA.

Typically (as I implement Enterprise Historians) I use DA for talking to PLCs and the like, and HDA for talking to Process Historians. However I'm aware that Rockwell in collaboration with OSIsoft are now providing a version of the PI Historian embedded at the PLC level - part of their FactoryTalk Historian suite. In such instances I would expect this historian to provide an OPC HDA server so that other systems could access the data.

Typically what we have found with HDA is some mapping of attributes is required between the client and the server. I only mention this as I haven't encountered this issue with the DA servers and clients I've installed.

Something to look out for on the install is getting the com and security settings correct. Typically vendors have their OPC HDA servers configured under a global admin account which makes them reluctant to give out passwords etc. The HDA Client will need to access the server using this account - we've had to get vendors to change the account passwords in the past where they have been reluctant to supply us with their default. This can carry costs either financially or to uptime.

Any questions then drop me a line, and I'll help where I can.

Regards,

Graham Burnikell
graham.burnikell@intuitive.uk.com