Ind. Code § 36-9-11-21

Sec. 21. If a pledge of revenues is made under section 20 of this chapter, then the revenues pledged shall be deposited monthly in the fund or account for the payment of the bonds. The pledge is irrevocable as long as any of the bonds for which the pledge is made are outstanding, but the municipality may not make any covenant relative to the number or location of the parking meters to be maintained by it in and along its streets.