I am implementing MODBUS protocol in C++. So far, I have implemented data link layer (Master/Slave) with State design patten. Currently, I have used RS-232 serial communication, but in future if TCP communication is chosen, then there is no need to change Master/Slave implementation.

However, my question is regarding Request message from Master.

Does master record information of memory allocations of all slaves (e.g. Starting Address, this address is part of Request message, sent from master to slave)?

Thank you

 

Your question is a bit ambiguous so I may not have interpreted your question correctly. However, the "starting address" refers to the coil/discrete input/register number which is the start of the block of coils/discrete inputs/registers that the command wants to read from or write to.

The client (master) sends whatever "starting address" the user (or application program) asked for. If any of the referenced addresses do not exist in the server (slave), the server will reply with an exception (error) rather than the data. It is up to the person using the software to know what addresses they want to ask for, as they are the only ones who know what their intentions are.