In SX Microcontrollers, SX/B Compiler and SX-Key Tool, George Herzog wrote:

If you can rely completely on SX/B to program, your PIC program would easily adapt.  The SX just creates the USART in software.  You won't see it mentioned in hardware documentation. 

As mentioned before, your SRAM registers are far smaller than the standard.  Also, since Modbus addressing is 16bits, not 8bits - you will possibly use up more registers for handling the addresses too.

Modbus seems very valid for supporting 'legacy systems'.  Apparently is was widely used in industrial applications.  There is even and Enron Modbus that I guess is for oil industry systems.

The SX alone can really only provide a 'mini-slave' due to the SRAM limit.  While the Modbus will support 256 nodes, RS-485 is generally limited to 32 nodes.  So you may not need a full Modbus register model.

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