# Derived Tag Write Expression throwing Bad\_ReadOnly when the source tag is not being referenced at all **URL:** \ **Category:** Ignition\ **Tags:** ignition81, vision\ **Created:** [June 6, 2023, 3:10am UTC](https://forum.inductiveautomation.com/t/derived-tag-write-expression-throwing-bad-readonly-when-the-source-tag-is-not-being-referenced-at-all/74609 "2023-06-06T03:10:18Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![sherman](https://avatars.discourse-cdn.com/v4/letter/s/e5b9ba/32.png) [@sherman](https://forum.inductiveautomation.com/u/sherman)\ **Post date:** [June 6, 2023, 3:10am UTC](https://forum.inductiveautomation.com/t/derived-tag-write-expression-throwing-bad-readonly-when-the-source-tag-is-not-being-referenced-at-all/74609/1 "2023-06-06T03:10:18Z")
I am still trying to build dynamic UDTs, and I am _so close!_ The only problem I have yet to overcome is how to use inferred values for referencing other tags, for writing. I can do it for reading with a small expression, but standard Expression Tags can't write. Derived tags at least _seem_ like they are designed to allow the expression language to write. After multiple iterations, I have something that looks like this (with the associated error if I manually change the `setpoint` value in the Tag Browser, which is how I test it): ![image](https://us1.discourse-cdn.com/inductiveautomation/original/3X/a/6/a6a9d717dd7f7c60840464006abe839ae8357a40.png) I understand that `{source}` is the text representing the path I want to reference, but there doesn't seem to be a dereferencing operator that does precisely what I want (like `float theVal = *_refTarget`). The `tag({source})` term effectively de-references the tag path stored in `_refTarget` (I know this because it works for reading out variables in other places), and the script "writeToByRef" is simply ```auto def writeToByRef(tag_path, value): system.tag.writeBlocking([tag_path], value) ``` I have confirmed this works in the script editor, using the text contained in my `[.]_refTarget` variable. Given the error that it can't write to the source, I am a bit confused as I thought I had hijacked the write expression to not even bother writing to the source tag. Is this a protective mechanism that maybe I can disable? Or maybe there is an actual way to do this directly in the expression language? ---
**Author:** ![pturmel](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/pturmel/32/3295_2.png) [@pturmel](https://forum.inductiveautomation.com/u/pturmel)\ **Post date:** [June 6, 2023, 12:14pm UTC](https://forum.inductiveautomation.com/t/derived-tag-write-expression-throwing-bad-readonly-when-the-source-tag-is-not-being-referenced-at-all/74609/2 "2023-06-06T12:14:03Z")
Don't write **within** the write expression. **Return** the value to be written. That's what expressions do--return values. The `{source}` given to the write expression is the **value** _written to your derived tag_, that you are responsible for converting as appropriate into a **value** suitable for the linked tag. Sigh. You are so hung up on object orientation that you refuse to use the platform as designed. ---
**Author:** ![sherman](https://avatars.discourse-cdn.com/v4/letter/s/e5b9ba/32.png) [@sherman](https://forum.inductiveautomation.com/u/sherman)\ **Post date:** [June 6, 2023, 7:21pm UTC](https://forum.inductiveautomation.com/t/derived-tag-write-expression-throwing-bad-readonly-when-the-source-tag-is-not-being-referenced-at-all/74609/3 "2023-06-06T19:21:28Z")
I'd rather do this! It's what I tried first, but I couldn't get past the "source tag is read only" error ~~(it isn't)~~ (the tag referred to by the path stored in `[.]_refTarget` isn't), but [.]\_refTarget is an expression (I want it to be a reference, but it doesn't seem to work, see below). It may also be because the \_refTarget is a path itself, not the actual tag, and I can't seem to dereference it. ---
**Author:** ![paul-griffith](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/paul-griffith/32/37249_2.png) [@paul-griffith](https://forum.inductiveautomation.com/u/paul-griffith)\ **Post date:** [June 6, 2023, 8:05pm UTC](https://forum.inductiveautomation.com/t/derived-tag-write-expression-throwing-bad-readonly-when-the-source-tag-is-not-being-referenced-at-all/74609/4 "2023-06-06T20:05:50Z")
With a 'Source Tag Path' of `[.]_refTarget`, which returns a string tag path, your read expression should just be `{source}` and your write expression should just be `{value}`. That's it. A derived tag is _already_ indirect. You don't need to use one for this purpose at all (a reference tag would suffice) - a derived tag is meant for _intercepting_ an outgoing value from the source tag and rewriting it in a more complex way than simple tag scaling can perform. The canonical example would be an OPC tag returning temp in celsius. A derived tag could be used for a _bidirectional_ temp-in-fahrenheit tag, by performing the appropriate conversion on read and write. No scripting necessary, and in fact scripting in derived tags is pretty much always a bad pattern, in my opinion. ---
**Author:** ![sherman](https://avatars.discourse-cdn.com/v4/letter/s/e5b9ba/32.png) [@sherman](https://forum.inductiveautomation.com/u/sherman)\ **Post date:** [June 6, 2023, 9:48pm UTC](https://forum.inductiveautomation.com/t/derived-tag-write-expression-throwing-bad-readonly-when-the-source-tag-is-not-being-referenced-at-all/74609/5 "2023-06-06T21:48:36Z")
So for clarity (because I did try this, and it doesn't seem to work as I hoped): `_refTarget` is an Expression Tag that builds a path based on the UDT parameters: ![image](https://us1.discourse-cdn.com/inductiveautomation/original/3X/7/5/7538230b01b594ef66a843d084509b27ef67e326.png) I want to use the resulting value as the target for my true IO, but I can't do it back through that because it is an Expression (read-only). If I try to change that to a Reference Tag (what I did initally), I get this: ![image](https://us1.discourse-cdn.com/inductiveautomation/original/3X/d/7/d7e84841544b8fc191a7629a2def6e4cf81f9bbe.png) this yields no resulting value, with a Quality of Uncertain\_InitialValue (irrespective of if I try to use a float or a string or an int -- the actual value at the example tag is a float). This _does_ yield a value in Expression mode, just not in Reference mode. Putting just `{source}` and `{value}` in my `setpoint` variable does not work -- if I do it in String type (the value should be a float), I get the tag path stored in `[.]_refTarget`, if I do it in Float, I get a type error (which makes sense if it is just getting the path. What I really want is to somehow do what feels like it should be `{[.]_refTarget}` in the Source Tag Path, but if I do that I get a `Error_Configuration`. I want the path stored at the address given! Same with the previous attempt -- if I try to do the following for my `setpoint` variable, I get an Uncertain\_InitialValue (not an error in config): ![image](https://us1.discourse-cdn.com/inductiveautomation/original/3X/8/f/8fef78718a415955a7f10e0f9e0999ce1823b519.png) ---
**Author:** ![sherman](https://avatars.discourse-cdn.com/v4/letter/s/e5b9ba/32.png) [@sherman](https://forum.inductiveautomation.com/u/sherman)\ **Post date:** [June 6, 2023, 10:01pm UTC](https://forum.inductiveautomation.com/t/derived-tag-write-expression-throwing-bad-readonly-when-the-source-tag-is-not-being-referenced-at-all/74609/6 "2023-06-06T22:01:58Z")
Here's a really simple instance of my problem -- just a toggle switch. I have a base type that holds some standardized metadata, I used that as the base UDT and then added a single variable `state` that should link to the tag whose path is stored in `_refTarget`: ![image](https://us1.discourse-cdn.com/inductiveautomation/original/3X/4/0/40ffcdc28adfa7d20c7650ad2a55e42abbfba357.png) I had this working somehow yesterday but I can't remember exactly how I did it (this one might have been on a previous less-generalized schema), but the Tag Browser deleted multiple of my UDT types somehow when I erroneously clicked and slightly dragged, so I am starting over anyway. This is an example of how I was successfully reading values, although it does not work for writing: ![image](https://us1.discourse-cdn.com/inductiveautomation/original/3X/6/f/6f0d2459c6f83544fa3b8772924089d36aeb264d.png) ---
**Author:** ![Craig\_Rice](https://avatars.discourse-cdn.com/v4/letter/c/87869e/32.png) [@Craig\_Rice](https://forum.inductiveautomation.com/u/Craig_Rice)\ **Post date:** [June 14, 2024, 2:12pm UTC](https://forum.inductiveautomation.com/t/derived-tag-write-expression-throwing-bad-readonly-when-the-source-tag-is-not-being-referenced-at-all/74609/7 "2024-06-14T14:12:20Z")
Kind of off topic but related. I have an UDT OPC tag where the value of the tag needs to be multiplied by 0.1. A derived tag works but then two UDTs need to be defined, one as OPC and the second as Derived do multiplication can be done. Is there a better way?