Difference between MODBUS protocol at layer 2 and layer 7 of OSI model


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I want to know the difference between Modbus at layer 2 (data link layer) and at layer 7 (application layer).

I have make one protocol convertor for temp monitor using serial interface RS-232 (Modbus to SCPI and vice versa protocol convertor). It is working properly.

I have use different function codes (01h, 03h, etc.) but now I have confuse that for which layer I have made application? On different sites I found many material. But all materials contain differece. Some says that it is leyer 2 application and some says it layer 7 application. So please help me as soon as possible. If anyone have material regarding this then please send me on my mail account: shah_devanshi22 @ yahoo. co. in