Bacnet MS/TP is a separately-licensed feature, requiring a feature named "mstp" in the JACE’s license file. For example:

Where port.limit defines the number of MS/TP trunks (JACE RS-485 ports) that can be used. This ranges from 1 to 4, varying on the type of the host JACE controller, and the license purchased.

The “EIA-485 (or RS-485) load factor” of connected MS/TP devices determines how many devices are physically supported, per trunk, due to electrical considerations. This ranges from 31 (“full load”) to up to 127 (“quarter load”) devices. Note that any other device limits in the license’s bacnet feature, or platform limits (e.g. JACE-403, 27 total devices) also apply.