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 idle. It is important to understand that if even one bus connected device is set to a valid baud rate (38,400, 76,800), this fixed idle state does not occur even when the NAE is removed.
Having a bus in a fixed idle state has some advantages for troubleshooting various problems. In this state, the bus should have very stable voltage readings so that a digital Volt-Ohm-Meter (VOM) or the FIT’s Meter, from the Voltage Menu , could be used to make sure all controllers are terminated properly and End-of-Lines (EOL) are set. If the FIT is connected to an MS/TP bus in a fixed idle state, it will display Offline. The FIT has the ability to start the token passing and request information from each controller on a bus. This feature is called Scan Bus. More information on this mode of operation is provided in the Scan Bus Menu section of this guide.
Once communication is occurring, the voltage on an MS/TP will change as communication occurs. A digital VOM in this state, can no longer provide reliable readings because of this fluctuation. If connected to a bus that already has communication occurring the FIT has the ability to merely listen, collect information, gather statistics, measure idle voltage readings when communication pauses or bit voltage readings when communication is occurring, and report bus health based on bus traffic and errors. If the FIT is connected to an MS/TP bus when communication occurring, it will automatically display the bus health. This listen-only mode of operation by the FIT is called Monitor Bus. More information is provided in the Monitor Menu section of this guide.
It is also important to understand MS/TP Bus installations can have Johnson Controls or Third-Party Vendor manufactured devices on them. Information availability, device responses, default baud rates, and voltage readings will vary from product to product and manufacturer to manufacturer. The information in this guide is based on the FIT’s use with current Johnson Controls manufactured products.
The FIT Display Table contains information for every potential FIT displayed item. The purposed of the table is to provide additional information about each FIT menu, selection or result.