Modern EPA requirements mean that it is impossible to design a compliant off highway commercial vehicle without relying on advanced electronic process controls. Your grandfather's tractor might be repairable with baling wire, but it also releases a ton more diesel particulates.

http://www.deere.com/en_US/ProductCatalog/FR/media/pdf/8r_se...

There's no incentive for companies to publish information on their vehicle bus interfaces, since it would basically give away trade secrets to competitors.