Engineering Reference
CAN, RS-232 and RS-485 Electrical Reference
Condition-aware electrical reference for CAN, RS-232 and RS-485 signaling, topology, termination, biasing, timing, cable capacitance, and interface boundaries.
- Reading Time
- 11 min
- Format
- Electrical-layer lookup
- Updated
- September 29, 2026
Standards and Device Limits Govern
This page is an orientation reference, not a compliance specification. Electrical limits, timing ranges, loading and topology requirements depend on the applicable standard edition, transceiver and controller documentation.
Interface Comparison
| Interface | Signaling | Signals | Typical topology | Boundary |
|---|---|---|---|---|
| CAN | Differential bus | CAN_H / CAN_L | Linear bus with termination at both physical ends | Nominal cable impedance and topology matter; protocol timing and transceiver electrical limits are separate |
| RS-485 | Differential multipoint | A / B naming is vendor-sensitive | Daisy-chain / linear trunk with short stubs | Polarity labels, unit loads, common-mode range and biasing require device documentation |
| RS-232 | Single-ended point-to-point | TXD / RXD relative to signal ground | Point-to-point | Logic sense and voltage levels differ from TTL/CMOS UART pins |
Termination and Cabling Terms
| Term | Implementation | Purpose | Boundary |
|---|---|---|---|
| End termination | Resistance near line Z0 at each physical end | Reduces reflections on a bidirectional differential bus | Adds driver load and power |
| Split termination | Two resistors with midpoint capacitor where supported | Can improve common-mode high-frequency behavior | Values and grounding depend on EMC design |
| Bias / failsafe network | Defines idle state when no driver is active | Avoids an undefined receiver input | Modern transceivers may include failsafe behavior; external bias changes loading |
| Stub | Branch from the main trunk | Connects a node | Keep short relative to edge rate and propagation delay |
| RS-232 cable capacitance | Capacitance per length × length | Loads the driver and limits edge rate | No single universal length follows from baud rate alone |
Canonical Calculation Anchors
| Case | Result | Interpretation |
|---|---|---|
| CAN: 16 MHz, BRP 2, 16 TQ | 500 kbit/s; TQ 125 ns | Controller convention must match the BRP definition |
| CAN: TSEG1 13 of 16 TQ | 87.50% sample point | Controller-valid segment and SJW ranges still apply |
| RS-232: 2500 pF budget, 50 pF/m cable | 50.0 m ideal capacitance budget | Connector and receiver capacitance reduce available cable length |
| RS-485: 120 Ω line and 120 Ω termination | Ideal resistance match | Correct resistance does not repair long stubs or poor topology |
Common Errors
- Connecting RS-232 voltage levels directly to MCU UART pins.
- Assuming RS-485 A/B polarity labels are universal.
- Terminating every node instead of the physical bus ends.
- Choosing termination without cable impedance.
- Ignoring termination current and resistor power.
- Treating bias resistors as termination.
- Using data rate alone to guarantee cable length.
- Ignoring edge rate and stub round-trip delay.
- Mixing CAN nominal timing with controller-specific register conventions.
- Ignoring common-mode and ground-potential limits.
- Treating a protocol connector as part of the electrical standard.
- Using typical values as compliance limits.
Support reference
FAQ
Are UART and RS-232 the same?
No. UART commonly describes asynchronous framing and logic-level signals. RS-232 defines an electrical interface with different voltage and logic conventions.
Are RS-485 A and B labels universal?
No. Vendor naming conventions can differ. Verify polarity from the transceiver datasheet and measured idle/active behavior.
Where should CAN termination be installed?
A conventional linear CAN bus is terminated at both physical ends, not at every node. Follow the network and transceiver specifications.
Does RS-485 always need two 120 ohm resistors?
No universal resistor value applies. Termination should match the actual cable characteristic impedance and topology, and short or low-speed links may use another documented strategy.
What is the CAN sample point?
It is the location within the nominal bit time where the controller samples the bus, commonly expressed as a percentage. Valid ranges are controller- and network-dependent.
Does baud rate alone determine RS-232 cable length?
No. Driver capability, receiver threshold, cable capacitance, noise, grounding and required margin also matter.
Why should RS-485 stubs be short?
A stub is a transmission-line discontinuity. Its round-trip delay relative to signal edge time can create reflections even when the nominal data rate is modest.
What is failsafe biasing?
It establishes a defined differential idle state when no external driver actively drives the bus. It must be coordinated with termination and transceiver features.
Can CAN and RS-485 share the same transceiver?
Not ordinarily. They have different physical-layer requirements and protocol use. Select a transceiver designed for the applicable standard.
Are these voltage values enough for compliance?
No. Compliance depends on the applicable standard edition, device tests, loading, common-mode range, timing, EMC and implementation details.
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