# Reading large block of data with OPCUA **URL:** \ **Category:** Ignition\ **Tags:** ignition80\ **Created:** [October 17, 2020, 7:11am UTC](https://forum.inductiveautomation.com/t/reading-large-block-of-data-with-opcua/39774 "2020-10-17T07:11:51Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![mori.luca](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/mori.luca/32/13424_2.png) [@mori.luca](https://forum.inductiveautomation.com/u/mori.luca)\ **Post date:** [October 17, 2020, 7:11am UTC](https://forum.inductiveautomation.com/t/reading-large-block-of-data-with-opcua/39774/1 "2020-10-17T07:11:51Z")
Hello, I was wondering about what is the best way to read large block of data from a plc, in my case Siemens s7-1500 (using integrated OPCUA Server). I have an UDT definition on my PLC looking something similar to this : ```auto MyUdt: Id --> int Name --> String(20) Code --> String(20) p1 --> Int p2 --> Int p3 --> long p4 --> bool p5 -->bool ``` The above UDT is instantiated 50 times in lets say DB5. DB5 starting from byte 0 has inside 50 UDT, not inside an array, just single UDT one after the other like this : ```auto DB5 : MyUdt_1 MyUdt_2 MyUdt_3 ... MyUdt_50 ``` It happens that I need to read all of them, to perform some logic and then write them back. At the moment, in order to perform a read action, I prepare my array of single tags, then i call the [system.opc.readValues](https://docs.inductiveautomation.com/display/DOC80/system.opc.readValues) and i have, after few seconds, the values i was looking for. I need to perform an OPC direct read as I can’t manage to subscribe to this tags, otherwise I will saturate the s7 Opcua Server. I remember using opensource DotNet DLLs to interact with Siemens PLC, and i was able to perform a block read lets say from DB5 byte 0 - 20000, and then when i had that array of bytes , i had to parse them to recreate the desiderate structure of data. From my understanding, this reading method should be more efficient. Correct me if I’m wrong, but event if i send a [system.opc.readValues](https://docs.inductiveautomation.com/display/DOC80/system.opc.readValues) instead of the system.opc.readValue, the request to the PLC is only one, but still the cpu has to search for every single tag i requested.(And i don’t find a way to register this Nodes on the Siemens OPCUA Server : [Registered reading of OPCUA server](http://forum.inductiveautomation.com/t/registered-reading-of-opcua-server/28160)) Is there any way, and if there is could you point me at some docs, to accomplish the same task? My aim is to read the DB5 data as quick as I possibly do, and give to Ignition the task to parse the data. Thank you for your time ---
**Author:** ![roland.scheuer](https://avatars.discourse-cdn.com/v4/letter/r/c57346/32.png) [@roland.scheuer](https://forum.inductiveautomation.com/u/roland.scheuer)\ **Post date:** [October 17, 2020, 8:31am UTC](https://forum.inductiveautomation.com/t/reading-large-block-of-data-with-opcua/39774/2 "2020-10-17T08:31:04Z")
Hi, what about the Siemens Driver ignition have? Br, Roland ---
**Author:** ![mori.luca](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/mori.luca/32/13424_2.png) [@mori.luca](https://forum.inductiveautomation.com/u/mori.luca)\ **Post date:** [October 17, 2020, 8:36am UTC](https://forum.inductiveautomation.com/t/reading-large-block-of-data-with-opcua/39774/3 "2020-10-17T08:36:47Z")
Hi, Thanks for your reply. My aim was to understand if there is a way to that same thing using OPCUA. Maybe something to do with XML files, or something like that. ---
**Author:** ![Kevin.Herron](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/kevin.herron/32/7259_2.png) [@Kevin.Herron](https://forum.inductiveautomation.com/u/Kevin.Herron)\ **Post date:** [October 17, 2020, 12:59pm UTC](https://forum.inductiveautomation.com/t/reading-large-block-of-data-with-opcua/39774/4 "2020-10-17T12:59:27Z")
When you read these 50 UDT instances are you requesting just the 50 top level UDT tags or are you requesting all of the member tags as well? ---
**Author:** ![mori.luca](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/mori.luca/32/13424_2.png) [@mori.luca](https://forum.inductiveautomation.com/u/mori.luca)\ **Post date:** [October 17, 2020, 4:54pm UTC](https://forum.inductiveautomation.com/t/reading-large-block-of-data-with-opcua/39774/5 "2020-10-17T16:54:49Z")
Hi Kevin, Sorry I think I misunderstood your question. At the moment I request all of the tags of each udt, so yes I’m requesting all members values. ---
**Author:** ![Kevin.Herron](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/kevin.herron/32/7259_2.png) [@Kevin.Herron](https://forum.inductiveautomation.com/u/Kevin.Herron)\ **Post date:** [October 17, 2020, 8:30pm UTC](https://forum.inductiveautomation.com/t/reading-large-block-of-data-with-opcua/39774/6 "2020-10-17T20:30:08Z")
That’s going to be much slower than requesting only each top level structured value instead, at least if you are using the OPC UA server built in to the S7-1500. When reading a structured value it will be received in Ignition as a JSON object, containing all the member values and any member structs as embedded JSON objects modeled the same way. ---
**Author:** ![mori.luca](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/mori.luca/32/13424_2.png) [@mori.luca](https://forum.inductiveautomation.com/u/mori.luca)\ **Post date:** [October 17, 2020, 8:49pm UTC](https://forum.inductiveautomation.com/t/reading-large-block-of-data-with-opcua/39774/7 "2020-10-17T20:49:35Z")
Ok thanks, I will try this approach instead. I imagine I can write the udt back to plc by passing as a value to be written a JSON right? ---
**Author:** ![Kevin.Herron](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/kevin.herron/32/7259_2.png) [@Kevin.Herron](https://forum.inductiveautomation.com/u/Kevin.Herron)\ **Post date:** [October 17, 2020, 9:09pm UTC](https://forum.inductiveautomation.com/t/reading-large-block-of-data-with-opcua/39774/8 "2020-10-17T21:09:22Z")
Yes, you just have to make sure you maintain the same overall structure. The UDT and every one of its members need to have Read Write permissions. If even a single member has Read Only permissions the write will fail. ---
**Author:** ![mori.luca](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/mori.luca/32/13424_2.png) [@mori.luca](https://forum.inductiveautomation.com/u/mori.luca)\ **Post date:** [October 17, 2020, 9:21pm UTC](https://forum.inductiveautomation.com/t/reading-large-block-of-data-with-opcua/39774/9 "2020-10-17T21:21:15Z")
Thank you very much Kevin, tomorrow I will try this out. ---
**Author:** ![mori.luca](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/mori.luca/32/13424_2.png) [@mori.luca](https://forum.inductiveautomation.com/u/mori.luca)\ **Post date:** [October 23, 2020, 12:33pm UTC](https://forum.inductiveautomation.com/t/reading-large-block-of-data-with-opcua/39774/10 "2020-10-23T12:33:08Z")
Hi Kevin, I did try the method you pointed out, and it drastically reduced reading time on my S7-1500 integrated opcua server. Before, using the opc.readvalues with the array of tags, i had the replay from the plc in about 35 seconds, reading something like 100 strings. Now, by reading the 50 udts i read the string i need, plus like 150 more values per udt so the amount of data being read from the PLC is way more than before, and i get my data back in something like 3 secs. Thank you for your time