Gentlemen, thank you for all the replies.
Let me clarify.
The machine has 6 heads each doing its task on command from the master on the RS485. It is built on a tractor, and the tasks are asynchronous, as it digs holes, among other things. Definitely 6 slaves. Each slave uses a C8051F310. Nice 8051, running multitasking as from "Realtime apps on small systems" from 8052.com, and a state machine.
The master is a C8051F120, and the board is too complicated, the processor is too complicated, and the program is too complicated. I would like to replace the master with another slave board, making everything easier, including spares. The F310 can handle the task, if the display is driven seperately. The master also has to control the tractor gears, accelerator, brakes, etc. It is not autonomous, there is a driver to hold the steering wheel, handle alarms, etc. The distance moved has to be controlled from the processor, not by the operator.
The SPI on the slave boards are available on a header, perhaps that is the way to interconnect to a display and keyboard driver board.
The GSM module can then also be on a seperate board, and be connected to the RS485.
Would that not make everything so much simpler?
Thank you.