Modbus RTU:
ModbusRTU is a serial communication protocol that connects different devices on the same network and would make the communication between them possible.
Modbus is transmitted over serial lines between devices. The cables used for the conecction are either RS-485 or RS-232 cables. The simplest setup would be a single serial cable connecting the serial ports on two devices, a Master and a Slave.
The data is sent as series of ones and zeroes called bits. Each bit is sent as a voltage. Zeroes are sent as positive voltages and ones as negative. The bits are sent very quickly. A typical transmission speed is 9600 baud (bits per second).
BACNet MSTP:
MS/TP is exclusive to BACnet and can be implemented using the EIA-485 signaling standard. This is a shielded twisted-pair (STP) LAN operating at speeds from 9.6Kbit/s to 76.0Kbit/s. This LAN type is particularly suitable for single controller and low cost communications.
Like the Control and Information Protocol (CIP) used in industrial application, BACnet uses objects to represent data on a network. Objects defined by the BACnet specification include Analog Input, Analog Output, Analog Value, Binary Input, Binary Output, Binary Value, Multi-State Input, Multi-State Output, Calendar, Event-Enrollment Program, Schedule, Command, and Device.
The BACnet protocol is a completely non-proprietary open communication software standard. The standard is now controlled by the ANSI (American National Standards Institute) and ASHRAE (American Society of Heating Refrigeration and Air-Conditioning Engineers) Standard 135- 1995. In June of 1995, ASHRAE adopted BACnet as a standard for the Building Automation industry.