Hi theary,
Are you developing Hybrid application or Native application?
If it is Hybrid application then open the jquery mobile look and feel html file. you can find something similar to be below input tag.
you can add event triggering operations within this tag. For example.. onClick will call "Operation1()"
"<input class="right" type="text" id="PReqNo" onClick=="Oeration1()" sup_html_type="text" sup_max_length="15" sup_num_of_decimals="0" readonly="readonly" /><span id="iS_Input_instance_PReqNo_help" class="help"></span>"
So you can define your Operation1 javascript function in custom.js, to define changes of other text fields.
Refer the below link for HTML input events.
JavaScript Functions - for javascript functions.
Native applications also has the event triggering operations, You have to refer the API list of the particular platform.
I hope this will helpful to you.
Regards
Yokesvaran Kumarasamy