Mitsubishi QJ71MB91 Modbus module

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
ch1_error_response_code	2	U16	1
ch2_error_response_code	4	U16	1
ch1_detailed_led_status	6	U16	1
ch2_detailed_led_status	7	U16	1
ch1_bus_message_count	3840	U16	1
ch1_bus_communication_error_count	3841	U16	1
ch1_character_overrun_error_count	3842	U16	1
ch1_message_discard_count	3843	U16	1
ch1_data_discard_count	3844	U16	1
ch1_failed_transmission_count	3845	U16	1
ch1_slave_message_count	3846	U16	1
ch1_slave_no_response_count	3847	U16	1
ch1_slave_nak_count	3848	U16	1
ch1_slave_busy_count	3849	U16	1
ch1_exception_error_count	3850	U16	1
ch1_communications_event_count	3851	U16	1
ch2_bus_message_count	3904	U16	1
ch2_bus_communication_error_count	3905	U16	1
ch2_character_overrun_error_count	3906	U16	1
ch2_message_discard_count	3907	U16	1
ch2_data_discard_count	3908	U16	1
ch2_failed_transmission_count	3909	U16	1
ch2_slave_message_count	3910	U16	1
ch2_slave_no_response_count	3911	U16	1
ch2_slave_nak_count	3912	U16	1
ch2_slave_busy_count	3913	U16	1
ch2_exception_error_count	3914	U16	1
ch2_communications_event_count	3915	U16	1

link settings as documented (unverified): RTU/ASCII; default 19200 8E1; FC 01,02,03,04,05,06,07,08,11,12,15,16,17,20,21,22,23; Supports both master and slave functions. Link operation function allows CH1 (RS-232) to communicate with multiple slaves on CH2 (RS-422/485).

bench facts as documented (unverified)

terminals	CH1: D-Sub 9-pin female, CH2: Detachable terminal block (M3 screws)
A/B polarity	SDA/SDB for transmission, RDA/RDB for reception
termination	RS-422: 330 ohm 1/4 W, RS-485: 110 ohm 1/2 W
shield/ground	Ground the shield at one end
wiring notes	RS-232 cable connector screw is M2.6
max registers/read	125
response timeout	30000 ms
broadcast	supported
protocol notes	Response monitoring timer default is 30 seconds (30000 ms). Broadcast delay default is 400 ms.
identification notes	FC17 (Report Slave ID) returns programmable controller CPU type data.
FC 43 device ID	not supported
power	5VDC internal current consumption: 0.31A
environment	Meets the general specifications in the user's manual for the CPU module used.
doc revision	SH(NA)-080578ENG-K

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

When the QJ71MB91 module is mounted on a remote I/O rack (MELSECNET/H remote I/O station), communication parameters are not automatically restored after a power cycle, requiring manual intervention via the utility package or explicit sequence programming to restart communication. [source]
confirmed: Baud rates, default baud (19200), framing (RTU/ASCII), and parity (E) — confirmed by Mitsubishi QJ71MB91 Specifications and User Manual
confirmed: Internal current consumption of 0.31A at 5VDC — confirmed by Mitsubishi QJ71MB91 Specifications
confirmed: Register map addresses (e.g., ch1_error_response_code @ 2, ch1_bus_message_count @ 3840, ch2_bus_message_count @ 3904) — confirmed by QJ71MB91 Buffer Memory List in User 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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