# How Ignition writes to an array tag **URL:** \ **Category:** Ignition\ **Created:** [May 6, 2020, 3:19pm UTC](https://forum.inductiveautomation.com/t/how-ignition-writes-to-an-array-tag/34484 "2020-05-06T15:19:46Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![NII](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/nii/32/12920_2.png) [@NII](https://forum.inductiveautomation.com/u/NII)\ **Post date:** [May 6, 2020, 3:19pm UTC](https://forum.inductiveautomation.com/t/how-ignition-writes-to-an-array-tag/34484/1 "2020-05-06T15:19:46Z")
If I have an opc array tag of 10 elements and I change one of the element, does Ignition write just this one element or does it write all 10 elements to the opc? In value-changed script of an array, when only one of the element’s value changes, does Ignition generate one event for the changed element or does it generate multiple events for all the elements in the array? ---
**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:** [May 6, 2020, 3:32pm UTC](https://forum.inductiveautomation.com/t/how-ignition-writes-to-an-array-tag/34484/2 "2020-05-06T15:32:16Z")
It will either write only the element or the operation will fail. Ignition writes to OPC array elements using the index range parameter of the Write service to target individual array elements. Some servers don’t support this and it fails. Some do and the write will succeed. If the server doesn’t support this parameter the only way to write is doing it yourself in scripting, where you have no choice but to write the entire modified array rather than an individual element. Tag change scripts deliver one event containing the entire array value each time the array value changes. If multiple elements changed at the same time the server would deliver that as one update to the array value, not multiple updates, and you’d see the same one update in your tag change script. ---
**Author:** ![kgamble](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/kgamble/32/8019_2.png) [@kgamble](https://forum.inductiveautomation.com/u/kgamble)\ **Post date:** [September 22, 2021, 6:14pm UTC](https://forum.inductiveautomation.com/t/how-ignition-writes-to-an-array-tag/34484/3 "2021-09-22T18:14:19Z")
I am trying to do the same thing, and am finding that writing directly to the OPC server works, but ignition is failing to write to it correctly with system.tag.write\_\_\_ This script will not write to the tag (though the `print(tag)` will print “TEST” it just won’t actually update the tag) ```auto tagPath = "[default]EXAMPLE_Array.1" system.tag.writeBlocking([tagPath], ["TEST"]) tag = system.tag.readBlocking([tagPath])[0].value print(tag) # prints "TEST", but does not actually change the tag ``` However going directly to the OPC path DOES work: ```auto server = "SIEMENS_TEMP" path = 'nsu=http://www.siemens.com/simatic-s7-opcua;s="MyFolder"."MyTest"[0,1]' returnQuality = system.opc.writeValue(server, path, "TEST") newValue = system.opc.readValue(server, path) print(newValue) # prints "TEST" and does actually change the tag ``` Is there something I am missing here? Ignition is flattening the 2d array into a 1d array of twice the length, and so I am accessing its indices in 1d in the first example, however in 2d in the second example. I appreciate any ideas! Edit: I am a dummy and I realized the first version is writing to a custom property! However, my question still stands. ---
**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:** [September 22, 2021, 6:51pm UTC](https://forum.inductiveautomation.com/t/how-ignition-writes-to-an-array-tag/34484/4 "2021-09-22T18:51:28Z")
Are you trying to write the whole array value with `system.tag.writeBlocking` or just to an element in it? I’m able to write whole arrays to an OPC server, but it does require Optimistic Writes to be enabled on the Tag Group to immediately see it with a subsequent `readBlocking` call for some reason. ```auto tagPath = "[default]Int32Array" print system.tag.writeBlocking([tagPath], [[1,3,5]]) tag = system.tag.readBlocking([tagPath])[0].value print(tag) ``` ---
**Author:** ![kgamble](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/kgamble/32/8019_2.png) [@kgamble](https://forum.inductiveautomation.com/u/kgamble)\ **Post date:** [September 22, 2021, 7:06pm UTC](https://forum.inductiveautomation.com/t/how-ignition-writes-to-an-array-tag/34484/5 "2021-09-22T19:06:47Z")
I noticed that with the value, I actually had a sleep in place to make it wait long enough to show However I am looking to write to specific elements ---
**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:** [September 22, 2021, 7:09pm UTC](https://forum.inductiveautomation.com/t/how-ignition-writes-to-an-array-tag/34484/6 "2021-09-22T19:09:13Z")
Okay, this will work as long as the server supports the index range parameter mentioned in earlier posts: ```auto tagPath = "[default]Int32Array[0]" print system.tag.writeBlocking([tagPath], [1]) tag = system.tag.readBlocking([tagPath])[0].value print(tag) ``` Still needs Optimistic Writes enabled. ---
**Author:** ![kgamble](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/kgamble/32/8019_2.png) [@kgamble](https://forum.inductiveautomation.com/u/kgamble)\ **Post date:** [September 22, 2021, 7:10pm UTC](https://forum.inductiveautomation.com/t/how-ignition-writes-to-an-array-tag/34484/7 "2021-09-22T19:10:37Z")
Ahh, so that’s what didn’t work when I tried I always got a “Bad\_Failure” with no reason Would it make sense for the server to support indexing elements through the OPC Path and not the range method you mentioned? It works there, just not through the Range version ---
**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:** [September 22, 2021, 7:11pm UTC](https://forum.inductiveautomation.com/t/how-ignition-writes-to-an-array-tag/34484/8 "2021-09-22T19:11:58Z")
> [@kgamble](#): > > Would it make sense for the server to support indexing elements through the OPC Path and not the range method you mentioned? Sure, this means the server has explicitly exposed each array element as a Node in the OPC UA address space. Whether a server does that and the syntax/NodeId used is up to the server.