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=item * Discrete outputs (or coils)

=item * Input registers

=item * Holding registers

=back

Note that discrete outputs are called coils as well. This name makes reference to the automation origin of the protocol, where a coil is the actuator of an electro-mechanical relay.

Tables may even overlay each other. For example, it is not uncommon to address a particular discrete input as a bit of a register address: bit 34 may be the 3rd bit of the 3rd input register. In this distribution, addressable tables are called I<zone...

To summarize, the Modbus data model breaks a I<unit> into I<zones> in which data is addressable.

Now, Modbus uses Protocol Data Units. The PDUs are the basic blocks defined by the protocol; they are the binary messages that flow between clients and servers. PDUs encapsulate a request from a client or a response from a server. They are independen...

PDUs are further encapsulated into Application Data Units, which add a header and (in the RTU case) a footer with further information as needed for the communication layer. Device::Modbus handles the RTU and the TCP variants of the protocol through L...

Finally, clients produce requests which are sent to servers, and they receive requests in return. This distribution provides the tools to build both servers and clients.

=head2 Request generalities