Text
Connector A --------+---------------- Transceiver A
                    |
                    +---- pin 1 (LINE1)

Connector B --------+---------------- Transceiver B
                    |
                    +---- pin 2 (LINE2)

Bus-side GND ------------ pin 3 (COMMON)

All three numbered pins belong to one MDD SM712 array.

1. Keep the normal line-to-ground voltage inside the +12 V / −7 V working envelope. Include inter-node ground offsets, biasing, and normal signal excursions in this check.
2. Check the protected transceiver's transient tolerance, not just its normal common-mode range. MDD specifies clamps up to +26 V and a 16 V negative magnitude at 17 A; a transceiver guaranteed only for normal +12 V / −7 V operation is not automatically protected at those stresses. Dynamic coordination between protection devices and bus pins is essential; see Bourns AN-1161.
3. No supply bypass capacitor, pull-up, or timing resistor is specified for the array itself. The MDD datasheet provides its internal circuit rather than a complete transceiver application schematic and gives no required external passive values. Decouple the transceiver according to its own datasheet; do not invent a capacitor requirement at the TVS.
4. Termination and failsafe bias belong to the RS-485 network. For a conventionally parallel-terminated 120 Ω cable, the usual arrangement is a 120 Ω resistor across A–B at each physical cable end, not at every node. Add external bias only when the selected receiver and required noise margin need it; calculate values from the actual supply, termination, and node loading. See TI's RS-485 Design Guide, sections 6–7.
5. For larger surges or sustained wiring faults, use coordinated additional protection. A TVS alone is not a fuse, current limiter, isolation barrier, or sustained-overvoltage solution. Select any primary surge protector and series current-limiting element together with the transceiver; do not copy values from an unrelated protection network. Bourns AN-1161 explains this coordination.

• Place the array at the cable-entry connector. Route each incoming conductor directly to its corresponding protection-pad junction before proceeding to the transceiver. Avoid long TVS stubs and, where possible, layer changes before protection. Keep sensitive unprotected traces out of the connector-to-TVS region.
• Make pin 3's return short and broad. Connect to a same-layer ground region with stitching vias immediately adjacent to the ground pad when an internal reference plane is available. Do not funnel the transient return through a narrow neck or a long shared logic-ground trace.
• Treat thermal relief as a solderability trade-off. Prefer a solid copper connection on the protection return where assembly permits; avoid narrow thermal spokes that add impedance. If relief is required for assembly, preserve adequate total copper width and validate the actual transient response. This is an engineering recommendation derived from the low-impedance-return objective, not an explicit MDD thermal-relief rule.
• Preserve differential-path balance. Route A and B together over a continuous bus-side reference, with similar protection junctions and minimal stubs. Set any controlled impedance from the interface/stackup design, not from the TVS package. Account for the verified 75 pF maximum zero-bias capacitance per protection path; no universal maximum baud rate was verified.
• Size transient copper from the actual pulse and board construction. Use short, broad line-to-TVS and ground connections; no universal minimum trace width is specified here. The 17 A number is a pulse limit, not a continuous-current routing requirement. Use the exact package drawing to check pad compatibility; any transceiver decoupling capacitor belongs close to that IC's supply pins, not to the passive TVS.

1. Confusing working voltage, breakdown voltage, and clamping voltage—or treating pulse ratings as continuous capability. The +12 V / −7 V envelope is not the surge clamp. At 5 A, the specified clamps are +20 V / −12 V magnitude; at 17 A, +26 V / −16 V magnitude. Include PCB overshoot and transceiver dynamic tolerance in qualification. Apply temperature derating: MDD's plotted rating is 40% at TA = 100 °C and 20% at 125 °C. Neither the 400 W figure nor the 17 A figure supports indefinite fault dissipation.
2. Assuming any “SM712” or any three-pin SOT-23 protection device is interchangeable. Verify manufacturer, pin numbering, polarity, package tolerances, clamping behavior, current waveform, capacitance, and temperature range. A wrong common-pin connection or long inductive return can defeat otherwise correct protection. The catalog's “7A 150W” discrepancy is an additional reason to use the exact supplied datasheet rather than the family name or library summary.

• MDD — SM712, “Asymmetrical TVS Diode for Extended Common-Mode RS-485,” revision 2024A2. Manufacturer datasheet content, hosted in the project's distributor-derived asset: 2407101112_MDD-Microdiode-Semiconductor-SM712_C502564.pdf. An exact publication day/month was not verified. Core ratings, pinout, electrical characteristics, package table, derating, and reel quantity are supported by inline citations above.
• Bourns — CDSOT23-SM712 Surface Mount TVS Diode, REV. 06/24. Manufacturer datasheet. Revision verified in the document.
• MCC — SM712 / ordering MPN SM712-TP. Official manufacturer product page; manufacturer datasheet endpoint. The product-page identity and functional category were checked; complete PDF-level specification and compatibility verification remains unresolved. Datasheet revision/date not verified.
• Texas Instruments — ESD Packaging and Layout Guide, Rev. B. Publisher application note. Sections 2.1–2.4 inform general PCB guidance; publisher-hosted text was reviewed, but independent PDF-level verification remains incomplete.
• Texas Instruments — The RS-485 Design Guide, Rev. D. Publisher application note. Sections 6–7 inform termination/failsafe guidance; publisher-hosted text was reviewed, but independent PDF-level verification remains incomplete.
• Bourns / Analog Devices — AN-1161, EMC-Compliant RS-485 Communication Networks. Publisher-hosted application note. Used for qualitative protection-coordination guidance; revision/date not verified. Its characterized circuits are not claimed as qualification of an MDD substitution.
• LCSC Electronics — MDD SM712, C502564. Exact seller listing. Used only for manufacturer/order identity, MOQ, reel quantity, and the dated availability snapshot; commercial terms may change.