When to use: Use when laying out a BACnet MS/TP trunk over RS-485. Each segment is limited to 32 unit loads (without a repeater), 127 master addresses (Max_Master), and 1200 m (4000 ft) of cable with EOL termination at both ends. As device count and baud rate change, the token rotation time changes — keep it under ~1 s for responsive control. Higher baud (38400–115200) supports faster rotation but shorter practical cable runs.
This tool calculates token rotation time, application data throughput, and device limits for a BACnet MS/TP segment running over RS-485, helping engineers design reliable field bus networks that meet ASHRAE 135 requirements. It is used when laying out controller trunks for VAV boxes, FCUs, and other field devices.
BACnet MS/TP is a collision-free master-slave/token-passing protocol defined in ASHRAE 135 Clause 9. Each octet on the RS-485 bus takes 11 bit-times (8 data + 1 start + 1 stop + 1 parity), so octet time = 11 / baud. Every master device must wait for the token to transmit; the token rotates through all master addresses up to Max_Master. Token rotation time = Devices × (data frame time + token frame time) + inactive address polling overhead.
At 38400 baud with 20 devices, a typical rotation time is 20–80 ms — acceptable for most HVAC control sequences. Application data throughput is the fraction of bus capacity devoted to payload octets after accounting for token passing overhead.
ASHRAE 135 (BACnet) governs MS/TP protocol behavior including token passing timers (Tusage_timeout = 20 ms default), Max_Master (0–127), and frame structure. The physical layer is EIA/TIA-485, which limits each unterminated segment to 32 unit loads (drivers + receivers) and a maximum cable length of 1200 m (approximately 4000 ft). Both ends of each RS-485 segment require 120-ohm termination resistors to prevent reflections. For practical reliability, most integrators limit segments to 16–24 devices even though the standard allows 32.
Keep token rotation time under 1 second for responsive control; above 1 second, supervisory reads become sluggish and alarms may be delayed. Use the highest baud rate compatible with your cable run length: 76800 or 115200 baud is fine for runs under 300 m, while 9600 baud may be needed for very long runs. Use shielded twisted-pair cable with the shield grounded at one end only to avoid ground loops. If more than 32 devices are required on a logical network, install an RS-485 repeater to create a second physical segment while keeping them on the same logical MS/TP token ring.
Select the baud rate and enter the number of master devices, Max_Master address, average APDU payload size, Tusage_timeout, and cable length. The tool reports token rotation time (keep under 1000 ms), application data throughput, bus unit-load percentage, and a cable length pass/fail check against the 1200 m EIA-485 limit. If token rotation exceeds 1 second, reduce device count, increase baud rate, or set Max_Master closer to the actual highest device address to eliminate polling of empty addresses.
EIA-485 limits a segment to 32 unit loads without a repeater. ASHRAE 135 allows Master addresses 0–127 (128 addresses), but physically fitting more than 32 requires RS-485 repeaters. Best practice is to limit segments to 16–24 devices for reliability and headroom.
ASHRAE 135 defines standard baud rates of 9600, 19200, 38400, 57600, 76800, and 115200 bps. 38400 bps is the most common field default. Higher baud rates allow faster token rotation but reduce maximum cable length due to signal rise-time constraints.
Max_Master defines the highest master address that the token-passing algorithm will poll for. Setting it higher than the actual highest device address wastes time polling empty addresses. For 20 devices addressed 0–19, setting Max_Master to 19 eliminates wasted polls and reduces token rotation time.
RS-485 termination resistors match the cable's characteristic impedance (typically 120 ohms for twisted pair) and absorb signal reflections. Without termination, reflections corrupt data at baud rates above 9600. Only the two physical endpoints of the bus should be terminated, not intermediate devices.
Not without a router. A BACnet MS/TP-to-IP router (often called a gateway or field controller) bridges the two data links. The router appears as a BACnet device on both networks and forwards messages between them using the BACnet routing protocol defined in ASHRAE 135 Clause 6.
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