It's total legalese, but here is the actual Illinois statute regarding non-resident income taxable by Illinois:
Compensation is paid in this State if:
(i) The individual's service is performed
entirely within this State;
(ii) The individual's service is performed both
within and without this State, but the service performed without this State is incidental to the individual's service performed within this State; or
(iii) Some of the service is performed within
this State and either the base of operations, or if there is no base of operations, the place from which the service is directed or controlled is within this State, or the base of operations or the place from which the service is directed or controlled is not in any state in which some part of the service is performed, but the individual's residence is in this State.
Bottom line: Illinois can only tax income you earn by performing services within Illinois. So technically they could tax any income you earn on your trips to Illinois. But, as TaxGuyBill stated, as a Texas resident Illinois cannot tax you on your earnings from physically working in Texas.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
