Client side functionality with server side values

Hi, I'm developing an SMS based application. Firstly I need to limit the user to 155 characters in a textbox,and secondly, he should see the number of available characters left to type increase/decrease as he types them. I have two HTML textboxes next to one another on a form. The one textbox is read/write and the other read only. User enters a value into the write textbox, and the read texbox must show how many characters are left. This is fairly straight forward javascript; I just used a calculation function that gets called through the onpropertychange event of the texbox. My problem comes in when I try and get the value of the texboxes throug C#. C# does not recognise the texboxes as they are client side, and if I run the controls as server controls, I lose the client side functionality of being able to show the user the character amount without having to do a post back Help?