Local FC Bus Rules and Specifications - BCPro - LIT-12011908 - General System Information - BCPro System - 4.0

MS/TP Communications Bus for the BCPro System Technical Bulletin

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BCPro System
Document type
Technical Bulletin
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Table 1, Table 2, and Figure 1 provide rules and specifications for the local FC bus.

Table 1. Local FC Bus Rules and Limitations




The BACnet Router can support one FC bus. Check BACnet Router product literature for more details.

CH-PC Controllers' EOL termination and bias circuitry are much more robust than BACnet Routers. We recommend using CH-PC Controllers at both ends of the FC bus.


An FC port on a BACnet Router can connect to only one bus segment on an FC bus. Only a daisy-chain topology is allowed (no T or Star topology configurations).

Number of Devices and Bus Segments

When one or more non-CH family device is connected on the FC bus, the device and bus segment limits are as follows: he device and bus segment limits are as follows:
  • 64 devices total per FC bus (maximum)
  • 3 bus segments per FC bus (maximum)
  • 32 devices per bus segment (maximum, not to exceed 64 devices per FC bus)

Bus segments on an FC bus are connected with repeaters (only). Up to two cascaded repeaters may be applied to an FC bus (to connect three bus segments).

Cable Length for FC Bus and Bus Segments

When all of the devices connected on the FC bus are CH-PCGs, CH-PCVs, or CH-PCXs, the cable length limits (using 22 AWG 3-wire twisted, shielded cable) are as follows:
  • Each bus segment can be up to 1,520 m (5,000 ft) in length.
  • Each FC bus can be up to 4,750 m (15,000 ft) in length.
When one or more non-CH family TEC26xx Series Thermostat Controllers or third-party MS/TP device is connected on the FC bus, the device and bus segment limits are as follows:
  • Each bus segment can be up to 1,220 m (4,000 ft) in length.
  • Each FC bus can be up to 3,660 m (12,000 ft) in length.
Note: Do not place subordinate devices on the FC bus. Subordinate devices on the FC bus are not supported.

When using fiber-optic connections: 2,010 m (6,600 ft) between two fiber modems.

Recommended Cable

22 AWG Stranded, 3-Wire Twisted, Shielded Cable

EOL Termination

The EOL switch must be set to On (or an EOL terminator installed) on the two devices located at either end of each bus segment on an FC bus. The EOL switches must be set to Off (or EOL termination disabled) for all other devices on the bus segment on an FC bus. See End-of-Line Termination on Local FC Bus for more information.

  • The recommended cable type provides the best bus performance. See MS/TP Bus Cable Recommendations for information on alternative cable types and lengths that may be used in MS/TP applications.
  • If third-party devices are connected to the bus, the cable lengths should be reduced (if necessary) to match the third-party vendor recommendations.
Figure 1. FC Bus with Three Bus Segments Connected with Repeaters

The bus segments on an FC bus are connected using repeaters (only). A repeater has two device connections, which are independent of each other. Each device connection on the repeater is connected to a bus segment just like any other device connection on the segment, and a repeater device connection can be connected at the end of a bus segment or anywhere along the segment. When a repeater device connection is at the end of a bus segment, EOL termination must be enabled on that repeater device connection. See Figure 1 for more examples of repeaters on FC buses.

Table 2. FC Bus Specifications



Error Checking

Message Headers checked using 8-bit Cyclic Redundancy Check (CRC) test. Message data check using 16-bit CRC test.

Device Addressing

0–255 (See Device Addressing on the MS/TP Bus for more information.)

Data Transmission Standard


Signaling Method


Signaling Rate

9600; 19,200; 38,400; or 76,800 baud as selected by bus supervisor. (See Baud Rates on the MS/TP Bus.)

Transient Immunity

Meets EN61000-4-4 and EN6100-4-5 requirements for heavy industrial applications.

Protected against misapplication of 24 VAC.

EOL Termination Method

Integral EOL Termination switch or add-on EOL Terminator module (See End-of-Line Termination on Local FC Bus.) Do not use third-party EOL termination.

Shield Grounding

Only one hard ground connection per bus segment when using shielded cable. (See Grounding the MS/TP Bus Cable Shield.)

Physical Configuration


Optional Vendor Components

  • Acromag® 4683-TTM-1F (115 VAC)

  • Acromag 4683-TTM-2F (230 VAC)

  • Acromag 4683-TTM-3F (24 VAC)
Note: A repeater is required to support more than 50 devices per trunk segment or trunk cable segment longer than 1,524 m (5,000 ft). Only the repeaters listed here provide EOL termination switching that is compatible with MS/TP.

Transient Eliminator

For further information, see Surge Protectors.

Fiber Modem
  • S.I. Tech Model 2110BAC Fiber-Optic Modem and S.I. Tech 2121 Power Supply

  • 9-pin Male Connector Kit (required by 2110BAC modem)
  • Does not support 76,800 baud
Note: A DIN rail mounting version of the Model 2110BAC may be available as Model 2110BAC-DIN. However it does not appear on the S.I. Tech web site at the time of this writing. Contact S.I. Tech using contact information on their web site at http://www.sitech-bitdriver.com/index.htm.