Understanding Modbus Terminology

Many of the object types are named from using Modbus in driving relays: a single-bit physical output is called a coil, and a single-bit physical input is called a discrete input or a contact.

Information is stored in the server device in up to four different tables. Two tables store on/off discrete values (coils) and two store numerical values (registers). The coils and registers each have a read-only table and read/write table.