Lochinvar KNIGHT/FTXL boiler

Register map(s) for this device, with a note of how far each has been checked. Addresses are 0-based, as on the wire. Scale and byte order are confirmed only at the hardware-verified rung.

coherence-checked

Round-trips through a real Modbus stack with no overlapping addresses and a word-order-sensitive codec — the map is internally coherent and wire-decodable. Does NOT prove byte-order or scaling match a real device: the emulator is seeded from the map's own types, so a uniformly wrong byte-order still round-trips. Only hardware-verified confirms byte-order/scale on the wire.

point	address (0-based)	type	scale	unit
rate_command_setpoint_command	2	U16	1	HEX
tank_setpoint	3	U16	0.5	°C
tank_temperature	4	I16	0.1	%
outdoor_temperature	5	I16	0.1	%
system_supply_temperature	6	I16	0.1	%
dhw_recirculation_temperature	7	I16	0.1	°C

link settings as documented (unverified): RTU; baud 9600/19200; FC 01,02,03,04,05,06,07,08,11,12,15,16,17,23; Baud rate selectable with Dip switch #9 (9600 or 19200 bps). Parity selectable with Dip switch #10 (No Parity with 2 stop bits, or Even Parity with 1 stop bit). Modbus address (1-247) set via DIP switches 1-8.

⚠ This manual must only be used by a qualified heating installer / service technician. Read all instructions, including this manual, the Installation and Operation Manual, and the Service Manual, before installing. Perform steps in the order given. Failure to comply could result in severe personal injury, death, or substantial property damage. — Starkest reads only; we never write a control register.

bench facts as documented (unverified)

terminals	A, B, GND Shielded
A/B polarity	A + (positive), B - (negative)
termination	Cable must be terminated with 120 ohm impedance matching resistor on each end.
wiring notes	Maximum Length = 4000 feet. Cable Specification = 24 AWG / A,B (twisted pair) and GND Shielded, with characteristic Impedance = 120 ohm. Maximum Load = 32 units (32 nodes).
response timeout	10000 ms
broadcast	supported
protocol notes	The timeout is adjustable between 5 and 120 seconds. The default timeout is 10 seconds. Each message is started by at least 3.5 character times of silence. The maximum delay between frames is 1.5 character times. BAS must refresh commands at least every 4 minutes.
doc revision	MODB-I-O_100161545_2000014822_Rev N

field reports — community-sourced, unverified (checked 2026-06-11)

To ensure proper operation, the configuration bits must be re-sent to holding register 40001 (or Object AV0) prior to issuing any command. If this is not done, the boiler may ignore subsequent commands. [source]
Integrators report difficulties writing to setpoints over BACnet/Modbus on Lochinvar boilers, sometimes requiring fallback to hardwired 0-10V setpoint control due to unreliable gateway behavior. [source]
confirmed: Baud rates [9600, 19200] and DIP switch settings (Dip switch #9 for baud rate, Dip switch #10 for parity, Dip switches 1-8 for address) — confirmed by Lochinvar Modbus and BACnet Communication Instructions manual (MODB-I-O_100161545_2000014822)
confirmed: Physical layer specifications (A/B/GND terminals, 120 ohm termination, 24 AWG cable, max 4000 ft, max 32 nodes) — confirmed by Lochinvar Modbus and BACnet Communication Instructions manual
confirmed: Polarity A + (positive), B - (negative) — confirmed by Lochinvar Modbus and BACnet Communication Instructions manual
confirmed: Supported Modbus function codes [1, 2, 3, 4, 5, 6, 7, 8, 11, 12, 15, 16, 17, 23] — confirmed by Lochinvar Modbus and BACnet Communication Instructions manual
confirmed: Timing and timeout quirks (adjustable timeout 5-120s, default 10s, 3.5 character silence, 4-minute command refresh) — confirmed by Lochinvar Modbus and BACnet Communication Instructions manual
confirmed: Register map summary (0-based addresses 2 to 7 mapping to holding registers 40003 to 40008) — confirmed by Lochinvar Modbus and BACnet Communication Instructions manual

The trust ladder

Each rung states what it proved and what it did not. Gray until proven; green is earned by hardware only.

generated — A prompt produced it. Unproven — anyone can regenerate this.
structure-verified — Decode path is structurally sound (addresses, widths, finite values). Does NOT prove word order or scaling.
coherence-checked — Round-trips through a real Modbus stack with no overlapping addresses and a word-order-sensitive codec — the map is internally coherent and wire-decodable. Does NOT prove byte-order or scaling match a real device: the emulator is seeded from the map's own types, so a uniformly wrong byte-order still round-trips. Only hardware-verified confirms byte-order/scale on the wire.
hardware-verified (software-key) x1 — Matched a real device with operator-attested readings as ground truth — the only rung that confirms word order + scaling on the wire.

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