PJON defines things up to level 3 -- you hand it a bunch of bytes, and tell it: "deliver to node 12". It is up to you to decide how the bytes must be interprete

PJON defines things up to level 3 -- you hand it a bunch of bytes, and tell it: "deliver to node 12". It is up to you to decide how the bytes must be interpreted, and to ensure the recipient can handle them properly. PJON has a number of data/physical link layers: a network running on a single arduino pin, link over LEDs, TCP, UDP, supercheap radio modules, RS485 serial. Compared to that, Modbus defines things up to application layer (layer 7). You tell it, "read input 1" or "write 75 to register 33" and it does it for you. You can also send arbitrary packets like PJON, but this is rarely done. Modbus also has number of physical layers: serial (usually used with RS485 or RS232), TCP, UDP, and high-speed proprietary links. It is not designed for low speed links, so it would not work well over LED link, supercheap radio modules, or a single arduino pin link.