Appendix A
MS/TP Communication Basics and the FIT
MS/TP is a token-passing (TP) communication protocol used in the building
automation and HVAC (Heating, Ventilation, and Air-Conditioning) industry that
allows devices to communicate with one another. The physical layer in which MS/TP
communicates is called RS-485. This physical layer consists of the hardware, including
address settings, cable, terminations, EOLs, baud rates, or shield grounds that allow
the MS/TP communication protocol to travel from controller to controller and other
devices.
To understand the data the FIT provides, it is important to have an understanding of
how MS/TP communication basically functions. MS/TP only allows for one controller
or device to talk at any given point in time. Communication is controlled by passing a
token from controller to controller based on address in a serial fashion (address 0 to
1 to 2 to 3 to 4). A controller or device must have its token to communicate on the
bus. Once its communication is complete, it passes the token to the next address.
This cycle repeats which each individual cycle is called a loop. The amount of time for
one loop is called Loop Time. The FIT displays this as looptime. It also provides other
useful bus statistics.
When a controller has the token, it transmits data in Frames. These Frames are
structured in size and content but ultimately contain the information being
communicated, the address of the communicating controller, or the address of the
destination controller.
For a loop to be successful, the physical layer has to be installed correctly. For
example, if you have two different MS/TP devices with the same address, the network
becomes unstable and produces unpredictable results. Both devices incorrectly
perceive that they received the token and they transmit outbound messages at the
same time. This causes communication collisions. The FIT attempts to let the user
know about any communication collisions or other problems with the physical layer.
See the FIT Issues Menu for more information.
For detailed information on BACnet® MS/TP Bus communication, installation, related
documentation, and cable guidelines refer to FAN-410 (File Access Number)
Installation Quick Reference Handbook (HVAC), Section 10 BACnet MS/TP
Communications Bus Guidelines (LIT-12011034), and FAN-410 Section 45 Metasys
System Cable and Wire Standards.
Most Johnson Controls manufactured devices are defaulted from the factory with a
baud rate of Auto Baud; however, the Network Automation Engine (NAE), as an
example, is defaulted to a baud rate of 38,400. This means that if no communication
is occurring, then the NAE starts a token at 38,400, whereas, in the absence of an NAE
or another device with a fixed baud rate, Johnson Controls devices wait for a valid
signal on the trunk before attempting to communicate. A bus in this state is called
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