we would like to know the modbus communication distance maximum.
Modbus itself specifies no distances, as it doesn't even specify any particular physical layer. Modbus can be carried over any media, such as RS-485, RS-232 etc., and the distance limitations would then be the ones particular to that physical layer.
If a 6600 used paper tape instead of core memory, it would use up tape
at about 30 miles/second.
-- Grishman, Assembly Language Programming