The two classic serial Modbus protocol flavours RTU and ASCII are implemented in the MbusRtuMasterProtocol and MbusAsciiMasterProtocol classes.
The popular vendor specific Extended Lufkin Automation Modbus Master (ELAM) protocol is also available as class MbusElamMasterProtocol. This proprietary Modbus extension allows adressing of up to 2295 slave units and the retrieval of up to 2500 registers for Modbus functions 3 and 4.
These classes provide functions to open and to close serial port as well as data and control functions which can be used at any time after a protocol has been opened. The data and control functions are organized into different conformance classes. For a more detailed description of the data and control functions see section Data and Control Functions for all Modbus Protocol Flavours.
Using multiple instances of a MbusRtuMasterProtocol or MbusAsciiMasterProtocol class enables concurrent protocol transfers on multiple COM ports (they should be executed in separate threads).
See sections The RTU Protocol and The ASCII Protocol for some background information about the two serial Modbus protocols.