Slave/Slave communication

Hi all,

I am working on a project that currently uses an RS-485 network with up to 32 slave devices (pic18f's with various IO and a max 483 driver) connected back to a master PC. The system is running fine, but it could be better.

The only time the master needs data from a device is when a change in the device occurs which is as low as every 30-40 seconds but normally is in the range of 10-20 minutes. When the change occurs, I need to log that time the master. To do this I am having the master poll each device in succession continually and as fast as it can (worst case 3-4 seconds/device if a timeout occurs). Based on this, I [I]could[/I] be logging event over a minute too late -not good.

Can anyone tell me how I can change my communication protocols so the slaves send data when it changes without first getting a request from the master? Like a slave only network? Does something like this already exist? I would bet so, but I haven't been able to find details about a reliable method of implementation.

I have have hardware in place and would like not change it if at all possible.
thanks
kevin