Tuya RS485 Probe Interface

Thought for 1 min 26 sec

flux

Production handoff is complete.

Corrected BOM includes the U2 exactly once.
Corrected pick-and-place includes its verified position: X −27.0 mm, Y 4.93 mm, 0°, top.
Visual orientation check passed: sensor/RS-485 holes and AC-input holes remain at opposite board ends.
Routing remains complete with 0 airwires and 0 redundant traces.

Use these corrected files instead of the original exported BOM and placement file:

Corrected BOM CSV

Designator	Quantity	Element UIDs	Part UID	Specification	Part Type	JLCPCB Part Class	Operating Voltage	Role	Role Details	Connectivity	Manufacturer Part Number	LCSC Part Number	Manufacturer Name	Datasheet URL	License	Package or Case Code	Voltage Rating	Mount	Resistance	Tolerance	Capacitance	Capacitor Type	Power Rating	Current Rating	Inductance	Symbol Style	Designator Prefix	Functional Group	Package
BR1	1	be2439e3-2f88-42de-ba95-003e401a0040	b542d48f-51a4-45dc-ba25-70f1b3e57ff1	Diodes, 4-SMD	Diodes			Bridge Rectifier	1 A, 100 V full-wave bridge; >2.8x expected 0.35 A AC source current		HD01-T		Diodes		https://creativecommons.org/licenses/by/4.0/	4-SMD		Surface Mount									BR	AC Input / Protection / Rectification
C1,C2	2	500a0ee7-8ade-42a5-80fb-89f37de61307,681e375e-7872-4167-9978-8621d1f0cc15	bfd6b918-378a-4370-8e2c-6531aada0868	Polarized Capacitor, 470uF	Polarized Capacitor			Bulk	Rectified DC bulk capacitor 1 of 2; total 940 uF for approximately 1.5 V ripple at 0.17 A DC load,Rectified DC bulk capacitor 2 of 2; parallel ripple-current sharing		EEV-TG1V471UQ		Panasonic Electronic Components		https://creativecommons.org/licenses/by/4.0/		35V				470uF						C	AC Input / Protection / Rectification
C3	1	7cb8d35f-1e1d-4fd6-b748-afceeeb7c97a	46707925-236f-106b-e676-afa2757aa1d6	Non-Polarized Capacitor, 100nF, SMD_0805_2012Metric	Non-Polarized Capacitor			Decoupling	High-frequency bypass on protected rectified DC bus		CC0805KRX7R9BB104		Yageo				50V			10%	100nF	Ceramic					C	AC Input / Protection / Rectification	SMD_0805_2012Metric
C4	1	b72c9cff-48a3-415f-a6f4-c1fb1d175c4e	46707925-236f-106b-e676-afa2757aa1d6	Non-Polarized Capacitor, 22uF, SMD_1210_3225Metric	Non-Polarized Capacitor			Decoupling	LMR33630 VIN ceramic input capacitor; 22 uF nominal, select 50 V X7R/X5R part with at least 10 uF effective capacitance at 20 V bias		GRM32ER71H226KE15L		Murata Electronics				50V			10%	22uF	Ceramic					C	3.3 V Buck	SMD_1210_3225Metric
C5	1	5be51e3b-da42-46ee-a908-cf79ce15b700	46707925-236f-106b-e676-afa2757aa1d6	Non-Polarized Capacitor, 220nF, SMD_0603_1608Metric	Non-Polarized Capacitor			Decoupling	LMR33630 VIN high-frequency bypass required by datasheet		CC0603KRX7R9BB224	C107083	Yageo				50V			10%	220nF	Ceramic					C	3.3 V Buck	SMD_0603_1608Metric
C6	1	3a1374fe-d573-4cdb-a077-6828189295ef	46707925-236f-106b-e676-afa2757aa1d6	Non-Polarized Capacitor, 1uF, SMD_0603_1608Metric	Non-Polarized Capacitor			Decoupling	LMR33630 internal VCC LDO bypass; datasheet-required 1 uF, 16 V ceramic		CL10A105KO8NNNC		Samsung Electro-Mechanics				16V			10%	1uF	Ceramic					C	3.3 V Buck	SMD_0603_1608Metric
C8,C9	2	5733437f-2929-4aa5-95b8-056fb55b1d36,f2b74d18-fb04-4904-9ac8-37680b3711d9	46707925-236f-106b-e676-afa2757aa1d6	Non-Polarized Capacitor, 22uF, SMD_1206_3216Metric	Non-Polarized Capacitor			Filtering	LMR33630 3.3 V output capacitor 2 of 2,LMR33630 3.3 V output capacitor 1 of 2		GRM31CR71A226KE15L		Murata Electronics				10V			10%	22uF	Ceramic					C	3.3 V Buck	SMD_1206_3216Metric
C10	1	a4ffe138-b5ff-4708-8b02-67594d92dedb	3382218a-0019-419c-ae13-6581c0420c04	Polarized Capacitor	Polarized Capacitor			Bulk	100 uF low-ESR shared 3V3 bulk; T1-U 445 mA maximum module allowance and 600 mA total rail planning allowance; retain with C11/C12 local bypass		EEH-ZC1E101XV		Panasonic Electronic Components		https://creativecommons.org/licenses/by/4.0/												CP	3.3 V Buck,Tuya Module / UI
C11,C13,C7	3	6ac7f8b6-6332-45c2-9451-ce802583da57,75513887-1ac4-4945-8d5b-b453b29a1694,f495b5f9-15ef-49e0-899f-780ac6f17a0c	46707925-236f-106b-e676-afa2757aa1d6	Non-Polarized Capacitor, 100nF, SMD_0603_1608Metric	Non-Polarized Capacitor			Bootstrapping,Decoupling	LMR33630 BOOT-to-SW bootstrap capacitor,MAX3485 VCC local decoupling,Tuya T1-U pin 14 3V3 local high-frequency decoupling; retain 100 nF		CC0603KRX7R9BB104	C14663	Yageo				16V,10V			10%	100nF	Ceramic					C	3.3 V Buck,RS-485 / Protection / Termination,Tuya Module / UI	SMD_0603_1608Metric
C12	1	8eef1a58-118b-4b58-9e8a-a24693436e44	46707925-236f-106b-e676-afa2757aa1d6	Non-Polarized Capacitor, 10uF, SMD_0805_2012Metric	Non-Polarized Capacitor			Decoupling	Tuya T1-U pin 14 local supply reservoir; retain 10 uF in parallel with C11 and 100 uF rail bulk C10		GRM21BR61A106KE19L	C77073	Murata Electronics				10V			10%	10uF	Ceramic					C	Tuya Module / UI	SMD_0805_2012Metric
D1	1	c1597f9e-28ad-48ec-bd48-a2da8bb72408	578f786f-38a8-4faf-90d4-d6e58f820091	TVS, SMA_L4.4-W2.6-LS5.0-BI	TVS	Extended Part		Protection	Bidirectional 24 V standoff TVS across fused 12 VAC input; avoids conduction at 14.5 Vrms high-line/no-load while clamping surges		SMAJ24CA-TP	C669029	MCC(美微科)			SMA_L4.4-W2.6-LS5.0-BI											D	AC Input / Protection / Rectification
D2	1	74ed4921-6822-4cb8-a644-2aa95c46aa32	7319d82b-aa7f-48ce-8889-d95e8df2d224	TVS	TVS			DC Surge Protection	24 V standoff TVS across protected rectified DC bus		SMBJ24A-TR		Taiwan Semiconductor Corporation														D	AC Input / Protection / Rectification
D3	1	06edaa96-9b92-4295-abec-b6c9222b3ecc	54caa3e1-5ab3-4589-a09b-fcde34733f24	Diodes, SOT-23	Diodes			RS-485 Surge Protection	SM712 asymmetric TVS array at outdoor probe connector		SM712-02HTG		Littelfuse		https://creativecommons.org/licenses/by/4.0/	SOT-23		Surface Mount									D	RS-485 / Protection / Termination
F1	1	1ee12cc0-afb3-4146-a146-9d1e5958ac2e	73fd9a9d-a4b7-4a41-ab6b-668c75625dbf	Fuse	Fuse			Protection	Littelfuse 1812L110/24DR, 24 V, 1.10 A hold / 1.95 A trip; 0.73 A hold at 60 C versus 0.49 Arms conservative source estimate; cURus/TUV		1812L110/24DR		Littelfuse Inc.		https://creativecommons.org/licenses/by/4.0/									1.1A			F	AC Input / Protection / Rectification
L1	1	fe889450-2003-4408-b796-acdbbaf9e833	70f35b39-cf68-4ee4-b836-6c316deb0ab4	Inductor, 2.2µH	Inductor			Energy Storage	Wurth 74438356022, 2.2 uH ±20%; calculated 1.07 A peak at 1.2 MHz worst case; 4.05 A Isat (10% L drop), 6.2 A thermal current, 35 mOhm max DCR		74438356022		Würth Elektronik		https://creativecommons.org/licenses/by/4.0/										2.2µH		L	3.3 V Buck
LED1	1	d19b0f24-1a24-4edf-aced-c9b4081f8b62	f18cc724-1d1a-41a3-8a5a-d84c4e098ec4	LED	LED			Power Indicator	Single green 3.3 V power LED		SML-D13FWT86C		Rohm Semiconductor		https://creativecommons.org/licenses/by/4.0/												LED	Tuya Module / UI
R1	1	08f21aee-4e15-48a5-8a56-e3f425489083	b67f50a9-ae5e-10ea-9c3c-d377e2ab3b2d	Resistor, 100kΩ, SMD_0603_1608Metric	Resistor			Feedback	LMR33630 feedback divider top resistor		RC0603FR-07100KL		Yageo				V		100kΩ	1%			0.1W				R	3.3 V Buck	SMD_0603_1608Metric
R2	1	325bd617-e6b9-48f8-8dbe-6574d489ea77	b67f50a9-ae5e-10ea-9c3c-d377e2ab3b2d	Resistor, 43.2kΩ, SMD_0603_1608Metric	Resistor			Feedback	LMR33630 feedback divider bottom resistor		RC0603FR-0743K2L	C137720	Yageo				V		43.2kΩ	1%			0.1W				R	3.3 V Buck	SMD_0603_1608Metric
R3	1	f6796e7a-4b34-4a0d-a56d-7b2febd9ea30	b67f50a9-ae5e-10ea-9c3c-d377e2ab3b2d	Resistor, 4.7kΩ, SMD_0603_1608Metric	Resistor			Pull Up / Down	RS-485 direction-control pull-down; keeps receiver enabled during Tuya startup GPIO uncertainty		RC0603FR-074K7L		Yageo				V		4.7kΩ	1%			0.1W				R	RS-485 / Protection / Termination	SMD_0603_1608Metric
R5	1	097586fd-052d-4d57-87d6-3719decdb78d	b67f50a9-ae5e-10ea-9c3c-d377e2ab3b2d	Resistor, 680Ω, SMD_0603_1608Metric	Resistor			Current Limiting	Power LED current limit; approximately 1.8 mA at 3.3 V with 2.1 V LED		RC0603JR-07680RL	C115415	Yageo				V		680Ω	5%			0.1W				R	Tuya Module / UI	SMD_0603_1608Metric
R6,R7	2	1e6aed24-6229-42ea-afab-826adb648645,e819a4cb-deef-44ef-bcf1-c82de4071451	b67f50a9-ae5e-10ea-9c3c-d377e2ab3b2d	Resistor, 10Ω, SMD_1206_3216Metric	Resistor			Protection	RS-485 B line surge-current limiting between transceiver and connector TVS,RS-485 A line surge-current limiting between transceiver and connector TVS		RC1206FR-0710RL	C108080	Yageo				V		10Ω	1%			0.25W				R	RS-485 / Protection / Termination	SMD_1206_3216Metric
R8	1	e88045a3-0c4d-4309-aa77-4f31f8db007c	b67f50a9-ae5e-10ea-9c3c-d377e2ab3b2d	Resistor, 120Ω, SMD_1206_3216Metric	Resistor			Impedance Matching	Optional RS-485 termination, enabled only by JP1		RC1206FR-07120RL	C114928	Yageo				V		120Ω	1%			0.25W				R	RS-485 / Protection / Termination	SMD_1206_3216Metric
R10,R9	2	9b05d42a-4310-4534-9a65-8adcfdd6c71f,c0d230e6-b54c-4b1f-8a5a-46a4b4c127db	b67f50a9-ae5e-10ea-9c3c-d377e2ab3b2d	Resistor, 680Ω, SMD_0603_1608Metric	Resistor			Biasing	RS-485 A fail-safe pull-up for master bus idle state,RS-485 B fail-safe pull-down for master bus idle state		RC0603FR-07680RL	C112310	Yageo				V		680Ω	1%			0.1W				R	RS-485 / Protection / Termination	SMD_0603_1608Metric
R12	1	09f4aa17-7f21-4697-aab8-d8aa66feb09e	b67f50a9-ae5e-10ea-9c3c-d377e2ab3b2d	Resistor, 100kΩ, SMD_1206_3216Metric	Resistor			Biasing	Rectified DC bulk-capacitor bleeder; approximately 94 s time constant		RC1206JR-07100KL	C136470	Yageo				V		100kΩ	5%			0.25W				R	AC Input / Protection / Rectification	SMD_1206_3216Metric
R13,R4	2	18c6034e-2a2a-4e78-8018-380ebe88b20b,bea0a829-63ae-45d4-b179-6f06820bb93b	b67f50a9-ae5e-10ea-9c3c-d377e2ab3b2d	Resistor, 10kΩ, SMD_0603_1608Metric	Resistor			Isolation,Pull Up / Down	10 kohm programming isolation: pin2 on RS485_RO_ISO from U3 RO; pin1 on UART_RX_RS485 shared by T1-U RX1 pin16 and J3 programmer TX pin3. Preserves programmer override without hard output contention.,Tuya P28 pairing input pull-up; button is active low		RC0603FR-0710KL		Yageo				V		10kΩ	1%			0.1W				R	Tuya Module / Programming,RS-485 / Protection / Termination,Tuya Module / UI	SMD_0603_1608Metric
SW1	1	bd14c69c-7a42-442d-a377-ecef39cb870b	c233f013-1b81-47a1-a2b1-de28e73cf1ad	Switch	Switch			Pairing Button	Momentary active-low Tuya pairing/reset input		B3U-1000P		Omron Electronics Inc-EMC Div		https://creativecommons.org/licenses/by/4.0/												SW	Tuya Module / UI
U1	1	ac0970a1-6c9a-46f9-823a-c6bc5e7909d2	6c0ac2ea-01af-431a-9ffd-c6687d52383b	DC-DC Converters, SOIC-8_L4.9-W3.9-P1.27-LS6.0-BL-EP2.8-1	DC-DC Converters	Extended Part		Buck Regulator	LMR33630 36 V input, 3 A synchronous buck; R1/R2=100k/43.2k sets 3.315 V nominal. Updated T1-U rail bound 519.4 mA; 600 mA design allowance. PCB/thermal and Wi-Fi transient verification deferred.		LMR33630BDDAR	C544370	TI(德州仪器)			SOIC-8_L4.9-W3.9-P1.27-LS6.0-BL-EP2.8-1											U	3.3 V Buck
U3	1	0af920c9-4fb9-4993-be03-77f235c4bfbb	9101dacc-2125-4732-bc2c-cab5c62c5abb	Integrated Circuit, SO-8_L4.9-W3.9-P1.27-LS6.0-BL	Integrated Circuit	Extended Part		Logic-level Shifting	MAX3485ESA+ 3.3 V half-duplex RS-485 transceiver; TX1 to DI, RO to RX1, P24 controls DE and active-low RE		MAX3485ESA+	C1545222	MAXIM(美信)			SO-8_L4.9-W3.9-P1.27-LS6.0-BL										parametric-v1	IC	RS-485 / Protection / Termination
U2	1		9c6c099d-9667-4f89-a14a-d2fbaa5014c9	Tuya T1-U Wi-Fi + BLE module	RF Transceiver		3.3V	Wireless MCU Module	Tuya T1-U, 3.3 V, provisioned firmware required	Wi-Fi + Bluetooth LE	T1-U		Tuya Smart	https://developer.tuya.com/en/docs/iot/T1-U-Module-Datasheet?id=Kc3qj1qy2phro		Module		PCB									U	Tuya Module / UI

Corrected pick-and-place CSV

Ref	Val	Package	PosX	PosY	Rot	Side
BR1		4-SMD	27mm	-1.7mm	0	Top
C1	470uF		20mm	0mm	180	Bottom
C2	470uF		2mm	0mm	0	Bottom
C3	100nF	SMD_0805_2012Metric	8.6mm	1.275mm	90	Top
C4	22uF	SMD_1210_3225Metric	6.3mm	1.275mm	90	Top
C5	220nF	SMD_0603_1608Metric	10.2mm	1.275mm	90	Top
C6	1uF	SMD_0603_1608Metric	19.75mm	-0.8mm	0	Top
C7	100nF	SMD_0603_1608Metric	19.75mm	1.275mm	90	Top
C8	22uF	SMD_1206_3216Metric	9.6mm	5.4mm	270	Top
C9	22uF	SMD_1206_3216Metric	11.9mm	5.4mm	270	Top
C10			-15mm	-1mm	0	Bottom
C11	100nF	SMD_0603_1608Metric	-22.5mm	-4.27mm	0	Bottom
C12	10uF	SMD_0805_2012Metric	-21.5mm	-2.4mm	0	Bottom
C13	100nF	SMD_0603_1608Metric	-8.7mm	0mm	90	Top
D1		SMA_L4.4-W2.6-LS5.0-BI	29.5mm	3.5mm	0	Top
D2			24mm	-6.8mm	0	Top
D3		SOT-23	-5mm	0mm	0	Top
F1			31.5mm	7.47mm	0	Bottom
L1	2.2µH		15.2mm	5.5mm	180	Top
LED1			3mm	5mm	0	Top
R1	100kΩ	SMD_0603_1608Metric	17.9mm	-4mm	180	Top
R2	43.2kΩ	SMD_0603_1608Metric	15.2mm	-4mm	180	Top
R3	4.7kΩ	SMD_0603_1608Metric	-32.4mm	-4.27mm	0	Bottom
R4	10kΩ	SMD_0603_1608Metric	-18mm	6mm	0	Bottom
R5	680Ω	SMD_0603_1608Metric	6.2mm	5mm	0	Top
R6	10Ω	SMD_1206_3216Metric	-6mm	4.7mm	0	Top
R7	10Ω	SMD_1206_3216Metric	-6mm	-4.7mm	0	Top
R8	120Ω	SMD_1206_3216Metric	-8.5mm	0mm	0	Bottom
R9	680Ω	SMD_0603_1608Metric	-8.5mm	3.3mm	0	Bottom
R10	680Ω	SMD_0603_1608Metric	-8.5mm	-3.3mm	0	Bottom
R12	100kΩ	SMD_1206_3216Metric	12mm	-8mm	0	Top
R13	10kΩ	SMD_0603_1608Metric	-20.8mm	1mm	0	Bottom
SW1			-14mm	6.8mm	0	Top
U1		SOIC-8_L4.9-W3.9-P1.27-LS6.0-BL-EP2.8-1	14.6mm	0.005mm	270	Top
U3		SO-8_L4.9-W3.9-P1.27-LS6.0-BL	-13mm	0.005mm	270	Top
U2	T1-U	Tuya T1-U castellated module	-27mm	4.93mm	0	Top

These should be submitted with the latest Gerber ZIP.

Files

Schematic

Layout

3D View

Project Specification

Purpose and scope

Proof-of-concept inline interface for a four-wire chemical sensor. The board accepts an isolated external Class 2 12 VAC source, supplies full-wave-rectified DC to a 9–24 VDC sensor, communicates by MODBUS-RTU over two-wire RS-485, and connects to Tuya cloud services through the exact Tuya T1-U Wi-Fi/Bluetooth LE module. The current migration covers the schematic only. Existing PCB placement, routing and fabrication exports are obsolete for the new module; do not manufacture before the later PCB review.

External connections

J1 — AC input: pin 1 = AC_IN_1; pin 2 = AC_IN_2.
J2 — sensor harness: pin 1 = RED / RECT_DC; pin 2 = BLACK / GND; pin 3 = YELLOW / RS485_A / T/R+; pin 4 = GREEN / RS485_B / T/R−.
Live schematic J1 and J2 are WIREPAD-2-1P6-6P0 and WIREPAD-4-1P6-4P0 bare solder-hole patterns, excluded from BOM and pick-and-place. Earlier Molex connector descriptions are obsolete; do not apply their agency claims to these wirepads.

AC input, rectification, and sensor rail

J1 feeds F1, now Littelfuse 1812L110/24DR: 24 VDC maximum, 1.10 A hold and 1.95 A trip. Its reference hold current at 60 °C is 0.73 A. The series has cURus and TÜV recognition.
D1, SMAJ24CA-TP, is a bidirectional 24 V stand-off TVS across the fused AC input. Its 26.7–29.5 V breakdown range avoids conduction during the stated 14.52 VAC RMS high-line/no-load case.
BR1, HD01-T, is a full-wave bridge with pins 2 and 4 as AC, pin 1 positive, and pin 3 negative. It is 100 V, 0.8 A average at 40 °C, and must be derated 20% for a capacitive load. It is UL Recognized, file E364304.
C1 and C2 are 470 µF, 35 V Panasonic EEV-TG1V471UQ capacitors in parallel. Each is rated 800 mA RMS ripple at 100 kHz and 125 °C.
D2, SMBJ24A-TR, is a unidirectional positive-bus TVS with cathode on RECT_DC, 24 V stand-off, 26.7–29.5 V breakdown, and 38.9 V maximum clamp at 16 A.
R12 is the 100 kΩ bleeder. J2 pin 1 receives the protected, filtered RECT_DC rail.

Under the updated 600 mA rail allowance in Power Budget, the planning sensor-rail envelope is approximately 11.17 V low-line trough to 19.13 V high-line/no-load peak, within 9–24 V under the inherited assumptions. Hardware verification remains mandatory because source impedance, bridge heating, capacitance tolerance and source regulation affect the result. The hot-environment F1 derating margin remains a system-level qualification item.

3.3 V buck converter

U1 is LMR33630BDDAR, recommended for 3.8–36 V input.
VIN and EN connect to RECT_DC; PGND and exposed thermal pad connect to GND; unused open-drain PG is intentionally no-connect.
C4 is 22 µF nominal, 50 V and must be sourced as X5R/X7R with at least 10 µF effective capacitance at 20 V DC bias. C5 is the required 220 nF high-frequency input capacitor.
C6 is 1 µF, 16 V, from VCC to GND. C7 is 100 nF from BOOT to SW. The VCC and bootstrap connections match the pin requirements.
L1 is now Würth Elektronik 74438356022, 2.2 µH ±20%, 35 mΩ maximum DCR, 4.05 A typical saturation current for 10% inductance drop, and 6.2 A thermal-performance current.
C8 and C9 are 22 µF, 10 V output ceramics. R1/R2 are 100 kΩ / 43.2 kΩ, yielding approximately 3.315 V nominal from the 1.0 V feedback reference.
The selected 1.4 MHz / 3.3 V support values follow TI's application table.
At 19.13 V input and 1.2 MHz minimum frequency, calculated nominal-inductance ripple is 1.034 A p-p and peak current is approximately 1.117 A at the updated 0.600 A rail allowance; allowing −20% inductance gives approximately 1.246 A peak.

Tuya T1-U support and exact firmware mapping

Manufacturer HTML documentation and its mechanical drawings are the sources; the PDF-query tool cannot index that HTML. The 445 mA maximum module current and UART1 flashing/power-cycle workflow are user-authoritative design requirements.

Supply: 3.0–3.6 V; design target 3.315 V nominal; module design allowance 445 mA.
Pin 14 3V3 → 3V3; pin 13 GND → GND. Retain C11 100 nF, C12 10 µF, C10 100 µF rail bulk, and C8/C9 22 µF each.
Pin 15 TX1 → U3 DI and J3 pin 4 (programmer RX) on UART_TX_RS485.
U3 RO → RS485_RO_ISO → R13 10 kΩ → UART_RX_RS485 → pin 16 RX1 and J3 pin 3 (programmer TX).
Pin 8 P24 → U3 DE and active-low RE on RS485_DIR; high = transmit, low = receive. Retain R3 4.7 kΩ pull-down.
Pin 17 P28 → SW1 active-low pairing button, with R4 10 kΩ pull-up. This is firmware pairing/reset, not CEN hardware reset.
Intentionally NC: pins 1 P14, 2 P16, 3 P23, 4 P22, 5 ADC, 6 RX2, 7 TX2, 9 P9, 10 P26, 11 P6, 12 P8, 18 CEN, 19 P21, 20 P17, 21 P15.
TX2 must float or be pulled high for normal application boot; never strap low. CEN has an internal pull-up and remains NC.
No CSN pin, R11, CBU_MODE, CBU_RESET or fictitious center EP is used in the T1-U schematic.
J3 remains exactly four holes, programmer perspective 3V3 G TX RX. No fifth CEN hole. Complete power discharge and prevention of UART back-power are required for the specified flashing workflow; verify it on hardware.
The UART guide warns that repeated power-pin cycling is not a universal recovery substitute for a 1 ms CEN reset. This limitation is accepted with the four-hole constraint; the guide itself does not document a flashing utility.

RS-485 interface

U3 is MAX3485ESA+, powered from 3.3 V and decoupled by C13.
Pin mapping is RO=1, active-low RE=2, DE=3, DI=4, GND=5, A=6, B=7, VCC=8; supply range is 3.0–3.6 V.
A is the non-inverting line and is routed through R6 to yellow J2 pin 3. B is the inverting line and is routed through R7 to green J2 pin 4.
D3 SM712 has pins 1 and 2 on A/B and pin 3 to GND. It is designed for RS-485's −7 V to +12 V common-mode range and 19 A 8/20 µs surge current.
R9 = 680 Ω pull-up on A and R10 = 680 Ω pull-down on B provide approximately 0.268 V differential idle bias with the optional 120 Ω termination enabled, exceeding the MAX3485 ±0.2 V receiver threshold.
R8 = 120 Ω is connected across A/B only when normally-open JP1 is solder-closed.

Human interface and counts

Exactly one power LED: LED1, SML-D13FWT86C, with R5 = 680 Ω, approximately 1.76 mA at a 2.1 V typical LED drop.
Exactly one pairing/reset button: SW1, Omron B3U-1000P, SPST-NO.

Validation status

See the T1-U migration review for current verification results. Earlier completion and fabrication claims are superseded. Stop at the schematic checkpoint; do not place or route the new module in this batch.

Purpose and scope
External connections
AC input, rectification, and sensor rail
3.3 V buck converter
Tuya T1-U support and exact firmware mapping
RS-485 interface
Human interface and counts
Validation status