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Concept

Concept


Products: Automation Server, AS-B, AS-P
Functionalities: Hardware
Product version: 2.0, 2.1, 3.0, 3.1
5/18/2018

Existing MNB Systems Transition

arrow1_rotationGuidelines on the Use of Legacy Cable Types

Many times the application of the EcoStruxure BMS 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 EcoStruxure BMS with the SmartX server picking up the existing MS/TP buses.

These expectations need to be examined from two perspectives:

  • Examine the compliance of the existing physical network installation with the recommendations provided regarding cable type, cable specifications, lengths, and network unit load. You can then make the best judgment if the installation should be considered suitable for expansion in cable lengths or node counts. Performing a network check-up is recommended. For more information, see Network Check-Up (Examination and Monitoring) . If the existing cable type or cable specifications are not compliant with the recommended cable, you may want to avoid committing to an expansion on the existing cable segment. The use of a spare MS/TP port on a SmartX server to start another MS/TP network, or addition of a SmartX server, may be a better choice.

Guidelines on the Use of Legacy Cable Types

This section lists guidelines to consider when you connect an EcoStruxure BMS and an existing MS/TP network that uses legacy cable types.

The existing MNB MS/TP networks typically use one of six cable types. These cable types are listed in the “TAC IA Series MicroNet BACnet Wiring and Networking Practices Guide” (F-27360-10). Five of the cable types are not compliant with the latest cable recommendations. For more information, see Cable Selection .

Table: Legacy Cable Types

Non-compliant

Compliant

Belden 864

Belden 89841

Belden 82641

Belden 82502

Connect-Air W241P-2000F

Connect-Air W241P-2000S

Belden 8641, 82641, and 82502

When encountering systems constructed using any of the cables Belden 8641, 82641, 82502, or other cables with equal or less performance (higher capacitance or unspecified capacitance or impedance), it is recommended that the network composition be maintained as-is with only the required transitions to connect with a SmartX server positioned in place of the previous head-end device at the same location. If expansion on the bus segment is needed, it is recommended that you stay within the reduced speed of 19,200 Baud or less and the reduced node count of 32 devices or less (as recommended in the “TAC IA Series MicroNet BACnet Wiring and Networking Practices Guide”). It is also suggested that a network check-up should be used to judge if any expansion should be performed.

Connect-Air W241P-2000F and W241P-2000S

The two cables Connect-Air W241P-2000F and W241P-2000S are cables that offer a lower cost cable while providing low capacitance specifications and a plenum rating. Both cable models have an impedance of 100 ohm.

The recommended cable list indicates preference for 120 ohm cable. For more information, see Cable Selection . This is the cable impedance recommended in the RS-485 standard and specified in the data for all of the RS-485 transceivers. The use of 100 ohm cable is a common practice thought to be directly related to the lower price and availability of several cable candidates. The use of 100 ohm cable is not optimum in regards to transmitter loading, noise immunity, and signal quality, but the diversion from the specified criteria is considered relatively small. The RS-485 interfaces have proven to be very robust and this is one of the areas where deviations from the standard frequently occur. The use of 120 ohm cable is recommended and may deserve more attention when expecting to operate over long distances (>600 m or 2000 ft) or environments of high noise induction.

It is commonly understood that data transmission cable should typically be terminated by a resistance equal to the characteristic impedance of the cable. The recommendations for Configuration 4 deviate from that guideline when suggesting the alternative termination resistance of 180 ohm when using 3.3 V bias source from the Automation Server. For more information, see MNB Configuration 4: Single End-point Bias (SmartX Server Provided Source) . The recommended configurations using 120 ohm termination are described as preferable, but the 180 ohm is an acceptable compromise for Configuration 4. A second deviation would relate to the use of 100 ohm cable. If 100 ohm impedance cable is used, it is recommended that the EOL termination resistance stay with the 120 ohm value as seen on the diagrams for Configurations 1, 2, 3, and 5. The use of a 100 ohm termination would reduce the idle line bias levels on the bus and further increase the load on the RS-485 transmitters.

Belden 89841

The Belden 89841 cable is fully compliant with the latest recommendations. For more information, see Cable Selection .

See Also

  • RS-485 Communications
  • MNB BACnet Devices
  • Network Check-Up (Examination and Monitoring)
  • Cable Selection
  • Architectural Guidelines 2.0.1, 2.0.2, 2.0.3, and 2.0.4
  • MNB Configuration 1: Dual End-point Bias (MNB Source)
  • MNB Configuration 2: Dual End-point Bias (External Supply Source)
  • MNB Configuration 3: Mid-point Bias
  • MNB Configuration 4: Single End-point Bias (SmartX Server Provided Source)
  • MNB Configuration 5: Single End-point Bias (MNB Device Provided Source)