# Prefix "C" not allowed **URL:** \ **Category:** General Discussion\ **Created:** [November 8, 2012, 6:54am UTC](https://forum.inductiveautomation.com/t/prefix-c-not-allowed/5388 "2012-11-08T06:54:25Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![cacordova](https://avatars.discourse-cdn.com/v4/letter/c/bbce88/32.png) [@cacordova](https://forum.inductiveautomation.com/u/cacordova)\ **Post date:** [November 8, 2012, 6:54am UTC](https://forum.inductiveautomation.com/t/prefix-c-not-allowed/5388/1 "2012-11-08T06:54:25Z")
Hi, I was creating a modbus address with a C prefix in ignition but for some reason it’s not allowing me. I tried adding the prefix T in the modbus addressing I didn’t have any problems. What could possibly be the cause of this? I’m only using a trial version of ignition could that affect it? Thank You ![:thumb_right:](https://us1.discourse-cdn.com/inductiveautomation/original/1X/2026e28c0c425034352c1b75807edf956adc75f4.gif ":thumb\_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:** [November 8, 2012, 2:58pm UTC](https://forum.inductiveautomation.com/t/prefix-c-not-allowed/5388/2 "2012-11-08T14:58:29Z")
HR, IR, C, and DI are all prefixes that are used when directly addressing a Modbus tag (in other words, directly creating the SQLTag in the designer rather than using the mapping configuration you’re using), and those prefixes aren’t allowed in a mapping to ensure the addresses don’t collide. ---
**Author:** ![cacordova](https://avatars.discourse-cdn.com/v4/letter/c/bbce88/32.png) [@cacordova](https://forum.inductiveautomation.com/u/cacordova)\ **Post date:** [November 8, 2012, 4:07pm UTC](https://forum.inductiveautomation.com/t/prefix-c-not-allowed/5388/3 "2012-11-08T16:07:23Z")
Thank You 😃