Concept
MNB BACnet Devices
This application note provides recommendations and guidelines for the configuration of RS-485 communications between the SmartStruxure server device and MNB BACnet devices. This information is intended to supplement the existing configuration documents such as the “TAC I/A Series MicroNet BACnet Wiring and Networking Practices Guide (F-27360-10)”. The guidelines focus on the arrangement of the electrical interface to the SmartStruxure server device's RS-485 port in regards to biasing, termination, cable selection, cable lengths and cable routing. The guidelines on maximum unit load (node count) and common mode voltage tolerance are associated with AS-P, AS-B, and Automation Servers with serial number of TD133954000 and later.
General MNB BACnet Device Properties
MNB BACnet devices use an RS-485 transceiver that provides integrated galvanic isolation (NVE IL3286E). This transceiver has an RS-485 unit load rating of 0.125 along with a weak bias resistance of 220 kohm creating a total unit load of 0.18. This is a little better (lower) than the 0.25UL listed in “TAC I/A Series MicroNet BACnet Wiring and Networking Practices Guide (F-27360-10)”.
For more information, see General MNB BACnet Device Properties .
MNB Configurations
The recommendations include five different configuration options to choose from with differences in performance and/or resources required.
For more information, see MNB Configurations .
Existing MNB Systems Transition
Many times the application of the SmartStruxure system will deal with existing MNB systems. In many of those instances, the existing MNB system installation may not be consistent with the network cable recommendations. For more information, see Cable Selection .
When dealing with an existing earlier installed system that has been operating for an extended period with no signs of problems, there will be an expectation that such a system can be effectively adapted to the SmartStruxure system with the SmartStruxure server device picking up the existing MS/TP buses.
For more information, see Existing MNB Systems Transition .
Network Check-Up (Examination and Monitoring)
When migrating an older system to a newer architecture or enhanced application environment, there is the possibility the existing system will encounter closer examinations, be expected to perform in a more detailed coordination with the larger system or be exercised in a more visible performance-sensitive manner by new applications. While an existing system may be considered stable and error free, the existing observations can sometimes be limited to the visibility offered by the earlier system. Communications problems can exist but go unnoticed (or masked) due by low level protocol retries. The typical MS/TP communications protocol setup for example can mask severe levels of communications errors with the retries not providing visibility of the issue until new dependencies on command/request latency reveals notable delays.
For more information, see Network Check-Up (Examination and Monitoring) .
Unit Load Definition, Maximum Network Load and Affects of Excess Unit Load
According to the TIA-485A standard, a single unit load is equivalent to a 12 kohm impedance attached to the + and – data lines (connected to ground or supply). A 1/8UL transceiver would have an impedance of 96 kohm. The TIA-485A defined total network load limit of 32UL is based on a common mode load resistance of 375 ohm connecting both the + and – data lines to ground (or CMV source). The standard requires the RS-485 drivers be capable of driving a network load of 32UL along with a Common-Mode Voltage (CMV) difference of -7 V to +12 V and produce a guaranteed minimum of 1.5 V transmit signal level. Such a full UL load with severe CMV conditions exhausts the maximum drive current of 60 mA provided by all standard RS-485 drivers. The specified minimum of 375 ohm resistance for the common mode load is the resulting resistance seen when 32 transceivers with 12 kohm input impedance are placed in parallel (12,000 / 375 = 32).
For more information, see Unit Load Definition, Maximum Network Load and Affects of Excess Unit Load (MNB and Generic RS-485 Devices) .
Expanded Unit Load with Network of Isolated Devices Only
If the network is comprised exclusively of devices with isolated RS-485 interfaces with the only exception being the SmartStruxure server device, it is recommended that the maximum unit load limit can be stretched higher. It is recommended that a maximum load extension should be 16UL (50% overload) giving a total expanded unit load limit of 48UL. Using a maximum network load of 48UL and subtracting the 24UL for the bias network and the SmartStruxure server device leaves 24UL available for the RS-485 devices. With the example device load of 0.18UL each, it is suggested that the isolated bus arrangement could support the full collection of up to 127 devices.
For more information, see Expanded Unit Load with Network of Isolated Devices Only (MNB and Generic RS-485 Devices) .
Cable Routing
The RS-485 network cable should be routed in a continuous daisy chain bus configuration. There should not be any stub connections, stars or ring configurations. The bussed cable should pass through each node to be connected with no splits or branches in the cable network.
For more information, see Cable Routing .
Cable Selection
This is one of the most important selections having significant impact on the performance and reliability of the RS-485 network being installed. An incorrect cable selection can be difficult and expensive to reverse. The decision should not be made on previous examples of seeing some alternate non-compliant cable work.
For more information, see Cable Selection .
RS-485 Communications
General MNB BACnet Device Properties
MNB Configurations
Existing MNB Systems Transition
Network Check-Up (Examination and Monitoring)
Unit Load Definition, Maximum Network Load and Affects of Excess Unit Load (MNB and Generic RS-485 Devices)
Expanded Unit Load with Network of Isolated Devices Only (MNB and Generic RS-485 Devices)
Cable Routing
Cable Selection