# Nested UDT and getBit **URL:** \ **Category:** Ignition\ **Tags:** ignition81\ **Created:** [May 8, 2025, 3:36pm UTC](https://forum.inductiveautomation.com/t/nested-udt-and-getbit/106052 "2025-05-08T15:36:54Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![msteele](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/msteele/32/77889_2.png) [@msteele](https://forum.inductiveautomation.com/u/msteele)\ **Post date:** [May 8, 2025, 3:36pm UTC](https://forum.inductiveautomation.com/t/nested-udt-and-getbit/106052/1 "2025-05-08T15:36:54Z")
I have a UDT let's call "Module" which contains the following... - 7+ nested UDTs let's call "Branch". - Bitfields/AlarmRegister, a bitpacked alarm register with alarms for each of the 7 branches. Within the nested branch UDT, I have a boolean expression tag with `getBit({relPathToContainingUdt/Bitfields/AlarmRegister},{branchNum})` to pull the correct alarm. This results in `Error_ExpressionEval` Is there no way to reference within a nested UDT to a tag in the containing UDT at runtime? ---
**Author:** ![Arnaud\_Declerck](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/arnaud_declerck/32/44872_2.png) [@Arnaud\_Declerck](https://forum.inductiveautomation.com/u/Arnaud_Declerck)\ **Post date:** [May 8, 2025, 3:55pm UTC](https://forum.inductiveautomation.com/t/nested-udt-and-getbit/106052/2 "2025-05-08T15:55:18Z")
hi @msteele The getBit expression take a number and a position as input. Therefor when you have the path to your alarm register you need to read the tag value with the tag expression. Here an example `getBit(tag(concat(replace({PathToParentFolder},{InstanceName},''),'AlarmRegister')),1)` be aware that is the bit into your branch udt in neeted into a folder structure, you will need to replace the instanceName but also the folder structure. 1 is the bit position you want to read. This can come from a parameter you set on each branch udt into your module udt. For example here i use my instance name to dynamically decide the position. you just need to make sure it's an integer. ```python getBit(tag(concat(replace({PathToParentFolder},{InstanceName},''),'AlarmRegister')),toInt({InstanceName})) ``` Here the result ![image](https://us1.discourse-cdn.com/inductiveautomation/original/3X/4/3/431d1d18b1caf26151d850f6169d1815b80c40ab.png) ![image](https://us1.discourse-cdn.com/inductiveautomation/original/3X/1/1/118b67d59e4f02f4415a71dcb247bc329bc5faf9.png) Hope it helps regards Arnaud ---
**Author:** ![msteele](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/msteele/32/77889_2.png) [@msteele](https://forum.inductiveautomation.com/u/msteele)\ **Post date:** [May 8, 2025, 4:28pm UTC](https://forum.inductiveautomation.com/t/nested-udt-and-getbit/106052/3 "2025-05-08T16:28:31Z")
Ah, that makes sense. I was hoping an extra `../` after the `{[.]` would get parsed within the instance and allow me to hop up to a reference in the containing UDT. I was trying to avoid using tag() and the performance hit. If I had to do that, I might as well just pull my bitmapped register to every nested instance, then use `getBit()` into the appropriate bit. Thanks for the clarification about relative tagPath behavior across UDTs!