I like it ! It is a useful change.

Could you refactor of Enum declaration to not break API compatibility:

enum ModbusRTUTxEnableMode: bool {
     /**
      * The TX enable pin should be high when transmitting
      */
     TxEnableHigh = true,

     /**
      * The TX enable pin should be low when transmitting
      */
     TxEnableLow = false
 };

Hi,
Rather than trying to not break ABI compatibility (not sure if Arduino libs can be released as pre-compiled binaries), I think that overloading the requests is more portable instead.
What do you think about the proposed change in the last commit?
Thx, L

To add my 2ct...
I think that "Direct" is a non-descript name and doesn't say anything about the logic involved. The fact that multiple users ask questions about it and that a full sentence is needed to describe it says enough.

I do like the first solution <0141b93>, but there are always more ways to achieve the same.

@lmartorella notice your ABI note v.s. API compatibility request. These libraries are source level, so ABI compatibility is never needed