So would you keep expanding the relay controllers by adding Slave Micros.

1- so many relays, so much Inductive load on PSU, so much noise......... 89x51 is too poor to handle that........

2- why don't you use simple serial to parallel conversion (STPC). As to what I understand from your posts is that you got some controlling Software on PC and that gives certain stream to your Master Micro from there you want to pull down or up a certain realy(s)..... if this is your scheme as I understand, then,

Simply use something like 4094 which connect like tail to head (or head to tail) each giving out 8 outputs so for 600 relays you would only need 75 of them. (each costing only Rs. 20, so cheap). All you would need to control the long distance cable noise problem if your relays are miles away from each other. And all 75 of them would need only 3 wires from one to the next to the next to the end.......

And you don't need a UART to drive them.

Good Luck

Babar