How to Select Isolated CAN and RS-485 Transceivers? BMS Communication Isolation Selection Guide
Bottom line: bus isolation comes down to three steps — first pick an integrated isolated transceiver by bus type (isolated CAN for CAN / CAN FD, isolated RS-485 for RS-485, isolated I2C for I2C) → then verify three hard specs (isolation rating and operating voltage, bus fault protection voltage and common-mode range, node count and ESD level) → finally add two companions (bus-port TVS + a secondary-side isolated auxiliary supply). This article uses Shenzhen HOPERF Microelectronics (HOPERF) CMT-series integrated isolated transceivers as the reference for the parameter comparison; all values follow the original manufacturer datasheet.
flowchart TD
A["① Determine bus type<br/>CAN / CAN FD / RS-485 / I2C"] --> B{"Rate and node scale"}
B -- "CAN FD 5Mbps" --> C["Isolated CAN<br/>CMT1042 / CMT1052"]
B -- "Classic CAN 1Mbps" --> D["Isolated CAN<br/>CMT1050"]
B -- "RS-485 long line, many nodes" --> E["Isolated RS-485<br/>CMT83085"]
B -- "On-board I2C sensor" --> F["Isolated I2C<br/>CMT810X"]
C --> G["② Check isolation rating and operating voltage<br/>5kVrms test value / ~1060Vrms operating value"]
D --> G
E --> H["② Check node count<br/>1/8 unit load = 256 nodes"]
F --> I["② Check level shift<br/>both sides 1.8~5.5V"]
G --> J["③ Check bus fault protection and common mode<br/>±70V fault / ±30V common mode"]
H --> K["③ Check ESD and fail-safe<br/>bus pins ±12kV / open keeps high"]
I --> L["③ Check direction control<br/>bidirectional auto-detect"]
J --> M["④ Companion: bus-port TVS + isolated auxiliary supply"]
K --> M
L --> M
M --> N["⑤ Layout: clearance under barrier<br/>independent decoupling on both sides"]First, the two most common questions
Q: Is it worth replacing a "digital isolator + normal transceiver" with an integrated isolated transceiver?
For new projects, generally yes. The integrated approach puts the isolation barrier and the physical-layer transceiver into one chip, roughly halving the board area and removing the TXD/RXD direction-control logic and fail-safe default-state alignment. Isolation rating, common-mode range, bus fault protection, TXD dominant timeout (DTO), over-current and over-temperature protection are all designed by the manufacturer inside the chip; when unpowered the bus presents an ideal high impedance and will not drag down the whole bus. The discrete approach's advantage is flexible channel direction and rate combinations and reuse of verified circuits — only necessary when the configuration is special or you are reusing an old design.
Q: Does 5kVrms mean it can withstand a 5000 V surge?
No. 5kVrms is the UL1577 isolation-withstand test value (about one minute of applied voltage without breakdown), describing insulation capability; a surge is a transient-energy event that must be handled by the isolator's own surge capability (typically 8 kV class for general isolators) together with a bus-port TVS for clamping. For the long-term operating voltage, check the datasheet operating isolation voltage (e.g. about 1060Vrms for HOPERF CMT1042).
Step 1: pick the model by bus type
| Bus type | HOPERF representative | Key specs | Typical scenario |
|---|---|---|---|
| CAN FD (5Mbps) | CMT1042 / CMT1052 | ISO 11898-2, ±70V bus fault protection, ±30V common mode, DTO dominant timeout | In-pack and inter-pack BMS comms, inverters, motor control |
| Classic CAN (1Mbps) | CMT1050 | 1Mbps, bus fault protection and over-temperature protection | Low-speed body and industrial CAN networks |
| RS-485 (half duplex) | CMT83085 | 12Mbps, 1/8 unit load 256 nodes, bus pins ±12kV system-level ESD | Smart-meter reading, PLC/DCS, industrial lines |
| I2C (on-board bidirectional) | CMT810X | Bidirectional auto-direction, both sides 1.8~5.5V level shift | Isolated sensors, EEPROM, metering chips |
Step 2: three hard specs you must verify
Spec 1: isolation rating and operating voltage
- Isolation withstand (test value): CMT1042 and CMT83085 are both 5kVrms class, in wide-body packages SOW8L / SOIC-16 / SOW16 — only a wide-body package gives enough creepage and clearance for system safety (e.g. IEC 62368-1, GB4943.1).
- Operating isolation voltage: this is the real basis for checking the system bus voltage and long-term cross-barrier voltage; CMT1042 is about 1060Vrms class. High-cell-count packs (e.g. 20S+ LiFePO4) must be checked with margin against the maximum bus voltage.
- Dual-side supply: CMT1042 is VDD1 2.5~5.5V / VDD2 4.5~5.5V; the two supply domains may differ, but the secondary side must come from an isolated auxiliary supply.
Spec 2: bus fault protection and common-mode range
Ground-potential drift between the pack and the chassis, and between modules, is the main cause of bus damage. CMT1042 provides ±70V DC bus fault protection and a ±30V common-mode range, meaning the transceiver still works rather than fails when the bus pins sit tens of volts off local ground. Size the margin at least 1.5x over the measured maximum common-mode drift.
Spec 3: node count and ESD level
| Parameter | Meaning | CMT83085 spec |
|---|---|---|
| Unit load | Sets how many transceivers a bus can hold | 1/8 unit load → up to 256 nodes |
| Bus ESD | Bus-pin ESD class | ±12kV system-level ESD (IEC 61000-4-2) |
| Fail-safe | Receiver output on input open/short | Output holds logic high, avoids false codes |
| Rate | Maximum data rate | 12Mbps |
The actual attachable node count must also deduct cable attenuation, termination and bias resistors, and the bus capacitance budget; for long distance or many branches derate further.
Step 3: two companions you cannot skip
Companion 1: bus-port TVS. The isolator handles common mode, the TVS handles transients — fitting only an isolator without a TVS lets lightning or hot-plug surge directly destroy the transceiver bus pins. The key for signal-port TVS is the trade-off between junction capacitance and bus rate: the higher the rate, the smaller the allowed junction capacitance. See Why RS485 and CAN Ports Keep Failing for the selection trade-off.
Companion 2: secondary-side isolated auxiliary supply. The isolator secondary side (bus side) must be powered by an isolated supply; taking it directly from the primary supply reconnects the primary and secondary grounds and voids the isolation. The isolated auxiliary supply is usually designed together with the on-board flyback; see How to Design an On-Board Auxiliary Supply (Chinese).
Design points and common pitfalls
| Position | Point | Common pitfall |
|---|---|---|
| PCB under barrier | No trace, no copper, no part on either side of the isolation gap | Traces or ground copper under the gap fail both safety and immunity |
| Decoupling both sides | 0.1μF close to VDD1 / VDD2 each | Only one side or too far from pins → false codes at high rate |
| Termination and bias | Per bus spec for termination and fail-safe bias | Missing RS-485 bias → false trigger when bus idle |
| Secondary supply | Independent isolated supply, no shared ground with primary | Secondary supply from primary → isolation shorted by the supply |
| TVS coordination | Isolator + signal-port TVS staged, surge clamped by front stage | Isolator only → surge destroys transceiver |
| Node budget | Calculate the ceiling by unit load with derating | Designed for full 256 at full load → unstable on long lines |
Related solutions
- Full isolator selection (digital isolators, gate drivers, isolated sampling): How to Select a Digital Isolator (Chinese)
- Bus-port TVS protection and junction-capacitance trade-off: Why RS485 and CAN Ports Keep Failing
- Isolator secondary-side supply design: How to Design an On-Board Auxiliary Supply (Chinese)
- High-voltage-box main loop and sampling protection: How to Select a TVS for BMS Protection Boards
- Full AMSEMI foldback TVS series and cross-reference: AMSEMI Foldback TVS: Full Series & Cross-Reference
- TVS principles, parameters and selection primer: What Is a TVS Diode?
- MCU and bus resource planning: Which MCU for a Storage BMS? GD32 Selection Guide (Chinese)
FAQ
Q1: What is the difference between an isolated CAN transceiver and a normal CAN transceiver plus a digital isolator, and which should I choose?
Both isolate the bus; the difference is integration and cost. The discrete approach is a multi-channel digital isolator plus a normal CAN transceiver — flexible in channel direction and rate combinations, and able to reuse existing parts, but it costs one extra chip and its surrounding parts, roughly doubles the board area, and you must align the TXD/RXD direction control and fail-safe default state yourself. An integrated isolated transceiver puts the isolation barrier and the physical-layer transceiver into one chip (for example HOPERF CMT1042 isolated CAN, CMT83085 isolated RS-485), removing the direction-control logic and roughly halving the board area; isolation rating, common-mode range, bus fault protection, dominant timeout and over-temperature protection are all designed by the manufacturer inside the chip. New BMS, inverter, meter and industrial bus projects generally prefer the integrated isolated transceiver; only fall back to the discrete approach when the channel configuration is special or you must reuse a verified discrete circuit. Final choice follows the system safety standard and the selected part datasheet.
Q2: What extra requirements does CAN FD at 5 Mbps place on the isolator?
The CAN FD data phase can run up to 5 Mbps, several times the 1 Mbps of classic CAN, so the isolator must be checked on three extra points. First, loop-delay budget: CAN FD sets an upper limit on the total transceiver loop delay (TXD to RXD) in the high-speed data phase, and the propagation delay introduced by the isolation barrier must be counted in that budget — look at the datasheet circular delay or propagation delay, not just the nominal rate. Second, bit-width symmetry and pulse-width distortion: the isolator pulse-width distortion compresses the usable bit time, so pick a part with low pulse-width distortion for the high-speed phase. Third, the rate class itself: confirm the transceiver explicitly supports 5 Mbps CAN FD rather than only 1 Mbps classic CAN — for example HOPERF CMT1042 / CMT1052 support 5 Mbps CAN FD, while CMT1050 targets 1 Mbps classic CAN. Also check the bus fault protection voltage (e.g. ±70 V) and common-mode range (e.g. ±30 V) for the ground-potential-drift scenario of high-cell-count packs. Follow the vehicle or equipment network spec and the part datasheet.
Q3: How many nodes can one isolated RS-485 bus carry, and what does 1/8 unit load mean?
The RS-485 node count is set by the transceiver unit load: a standard transceiver is 1 unit load and a bus holds at most 32; a 1/8 unit-load transceiver has eight times the input impedance of the standard type, so the same bus can hold up to 256 nodes. The HOPERF CMT83085 isolated RS-485 transceiver is 1/8 unit load and supports 256 transceivers on one bus, suited to dense-node scenarios such as smart-meter concentrator reading, PLC/DCS communication modules and industrial production lines. The actual attachable count must also deduct cable attenuation, termination and bias resistors, and the bus capacitance budget; for long distance or many branches the node count must be further derated. Final count follows on-site measurement and the transceiver datasheet bus-load table.
Q4: If the bus already has an isolated transceiver, do I still need a TVS protection device?
Yes — they cover different duties. The isolated transceiver solves the ground-potential-difference and common-mode-voltage problem: it electrically separates the bus side from the MCU side so that tens of volts or even kilovolts of common-mode drift do not enter the control circuit; its isolation rating is insulation capability, not surge withstand. The TVS solves transient overvoltage energy clamping: lightning, inductive-load switching, hot-plug and ESD transients must be clamped by a bus-port TVS to a level the transceiver can survive. They are complementary — tolerate common mode vs. survive the transient — and fitting only an isolator without a TVS lets surge directly destroy the transceiver bus pins, while fitting only a TVS without isolation still lets common-mode drift damage the downstream circuit. For signal-port TVS, weigh the junction capacitance against the bus rate; see the RS485 / CAN port protection article on this site.
Contact us
For HOPERF CMT-series isolated transceiver (isolated CAN / RS-485 / I2C) selection review, cost-down evaluation of replacing discrete solutions, sample requests and certification documents, contact us.
Shenzhen Intek Technology Co., Ltd — electronic component distributor and system solution provider Phone / WeChat: 136-3264-8484 Address: 4F, Building 2, Jingwei Center, No. 309 Ping an Avenue, Pinghu Street, Longgang District, Shenzhen Website: www.intek.vip
Disclaimer: This article is technical education and industry exchange for reference only and is not selection, procurement or other commercial decision advice. The isolation withstand, operating voltage, common-mode range, fault protection voltage, node count, ESD level and rate parameters are taken from HOPERF official public materials (product pages and datasheets) as typical values or class examples; they vary significantly by model, package and operating condition. Actual design must follow the selected part original datasheet, system safety standard and on-board measurement.