"As a side issue - have you considered the safety aspects of what you are proposing? What happens if your code goes west and tells the tractor to go full throttle? "

Yes. To the extent possible this issue was addressed.

1. If the UHF comm link fails, the reciever issues an output that interrupts the MCU initiating a Stop procedure. ( This takes care when the tractor just runs out of the communication range or looses signal someother way)

2. An independent electronic watchdog ( powered by a seperate battery and not the tractor battery )coupled with a bit from the MCU keeps a relay from dropping off when the tractor is moving. The moment this relay drops, the throttle circuit goes dead and defaults mechanically to idle and the spring loaded clutch actuator disengages and a similiar spring loaded brake actuator clamps.

Thats it. So far we have not had any runaway situation. And I am fully aware that there might be a situation that can still be a loop hole.

( Actually during the initial try there was this enthusiastic engineer who was running behind the tractor trying to read some warning messages on the vehicle mounted display unit. ( OK the control unit was kept on the driver seat and thus he could not sit on it - not that he was keen either )Suddenly the guy handling the remote switched off the Power and the tractor just stopped dead - the running engineer had the shock of his life, and was all over the rear of the tractor.)

Raghu