Specifications

Trunk Number
On a supervisory engine, trunk numbers 1 and 2 are reserved for the local field bus integration. Trunk numbers 3
to 20 are reserved for Remote Field Bus integrations. The quantity of field buses supported on any particular device
varies according to the specific device in use. Generally, the number of supported trunks is less than the maximum
trunk number allowed (Table 3). For example, an ODS supports 16 field buses maximum, not 20 field buses as the
trunk number range would suggest. If the specific device does not support a local field bus, such as an NxE85, start
with trunk 3, as trunk numbers 1 and 2 are not supported.
Network Address
The network address you specify for a Remote Field Bus integration must match the BACnet Router MS/TP-side
network number you specify. The network address must be unique at the site. Keep in mind that each MS/TP trunk
on each NxE has a network number that needs to be considered when determining a unique ID. This address is
used to locate and establish communication between the Remote Field Bus and the router. A defined Remote Field
Bus can only work with one router. If you need to change the network address of a Remote Field Bus after it has
been configured, you must restart the Metasys supervisory engine to complete the re-configuration.
Number Of Field Buses Allowed
The total number of local and remote field buses allowed depends on the type of controller as outlined in Table 3.
For example, an NIE55 can have up to eight field buses, but an NCE25 or NIE29 can only support one local field
bus and no remote field bus.
Table 3: Maximum Number of Field Buses per Controller
Maximum Number of Field Buses (local and remote)Controller
8NAE55 / NIE59
4NAE35 / NAE45 / NIE39 / NIE49
1 (local field bus only; Remote Field Bus not supported)NCE25 / NIE29
16NAE85 / NIE89
16ADS / ADX / ODS
Note: The Remote Field Bus supports up to 32 field devices per BACnet Router on a bus with all Johnson Controls
devices, or up to 16 field devices on a bus with a mix of Johnson Controls and third-party devices. Limitations
are listed in the Metasys System Configuration Guide (LIT-12011832). Also, to take advantage of bus
diagnostics, at least one Johnson Controls field device is required on the Remote Field Bus.
Configuration Attributes Only For Local Field Bus
If the same field bus configuration attribute is specified at both the BACnet Router and the Remote Field Bus, the
attribute setting on the BACnet Router prevails. For example, if a Baud Rate of 38400 is specified at the BACnet
Router but a Baud Rate of 76800 is specified on the Remote Field Bus, 38400 is used as the Baud Rate on the bus.
To be consistent, we recommend that you match the Remote Field Bus attribute values with that of the router, or
you can keep the default values at the Metasys supervisory engine. See BACnet Router Configuration for a partial
attribute list.
BACnet Router Wiring
Figure 5 shows how to wire the BACnet Router to the Metasys system. This example features the Loytec Router,
Model LIP-ME201™, the recommended device from Loytec Electronics GmbH.
13MS/TP Communications Bus Technical Bulletin