You can squeeze modbus (rtu or ascii) with some extra logic into a 89c2051 comfortably in assembler, allow about 5k if you use 'c' -
Squueze ??? I agree that Modbus is an excellent bus for many applications; however, if you are memory conscious you can fit a 485 protocol into <500 bytes.
If you are "talking" to equipment made by others, you will, of course, have to adhere to whatever bus they use, but if you are solely "talking to yourself" I would still use RS485 sice a roll our own protocol can be tailor made to your particular application. As an example I redid an "internal comm only" product from a standard bus to a custom 485 bus and gained better then 800% throughput improvement without sacrificing any checks.

Again, ABSOLUTELY nothing wrong with using a standared protocol when that coice makes the most sense.

Erik