AHS36 / AHM36 - What have to be considered by establish a CANopen network?

Purpose: Provide a compact, technical checklist for designing a CANopen network with SICK AHM36 encoders, clarifying trunk vs. stub lengths, the bit‑timing / arbitration reason behind length limits, and how to choose 125 / 250 / 500 kbit/s vs. 1 Mbit/s for robustness.

Reference documents for this article:

1. SICK Manual – Table 141 (“Maximum stub line length”)

2. Bus length vs. bitrate table (attached)

3. CiA CANopen CC bit timing table (bus length + max/ accumulated stub length)

4. Example network graphic (20 m trunk, 5 m stub, 2×AHM36)

1) Encoder capability vs. network capability (common misunderstanding)

• Encoder capability: The AHM36 CANopen supports high baud rates including 125 / 250 / 500 kbit/s and 1 Mbit/s. Therefore, from the encoder perspective, these baud rates are allowed (among others).
• Network capability: Whether 1 Mbit/s (or any high baud rate) works reliably is primarily determined by the physical network: topology, termination, trunk length, stub lengths, EMC / grounding. The CiA CANopen physical-layer guidance explicitly treats these as design constraints.

Engineering rule: If 500 Kbit/s meets the bus‑load / performance need, prefer 500 Kbit/s over 1 Mbit/s because timing / reflection margins are higher at lower baud rates. Simply, at lower baud rates the network robustness is higher.

2) Two different “length” terms (must be separated)

1.Trunk / Bus length (end‑to‑end)

• Definition: The length of the terminated main line between the two physical ends with 120 Ω termination.
• Where used: The attached bus length vs. bitrate table and CiA bus-length guidance.

2. Stub / Drop‑line length (unterminated branch)

• Definition: The unterminated branch from the trunk to a node (often via T‑connector). CiA explicitly defines stub / drop cables as an unterminated branch off the CAN cable.
• Where used: SICK Manual Table 141 (device-specific stub limits) and CiA “max stub length / accumulated stub length”.

Practical implication: A project can meet “bus length” guidance but still fail if stubs are too long (reflection-driven errors). Therefore, keep stubs as short as possible.

3) Why bitrate limits trunk length (bit‑timing / arbitration)

• CAN arbitration is bitwise; nodes must observe the bus and resolve dominance within the timing window of a bit. As bitrate increases, bit time decreases, leaving less margin for signal propagation (cable delay) plus device delays. This is the technical basis for the attached bus-length-vs-bitrate table.
• CiA’s CANopen CC bit timing guidance explicitly connects bit timing (including sample-point placement) to resulting maximum network length.

4) Why stubs limit high baud rates (reflections)

• A stub is an unterminated branch, it acts as an impedance discontinuity and creates reflections. At high baud rates, reflected edges can interfere with sampling, causing bit errors, error frames, and possibly bus‑off behavior.
• CiA 303‑1 recommends keeping stub cables “as short as possible” and provides engineering rules-of-thumb for single and cumulative stub lengths.
• CiA “CANopen lower layers” provides explicit max stub length and accumulated stub length per baud rate, reflecting this reflection / timing constraint.

5) Hardware & installation requirements (practical setup checklist)

5.1 Topology

Use line topology (trunk) with T‑connectors and short stubs. Avoid star topologies and long branches.

CiA 303‑1 explicitly supports device connection via T‑connectors or stub cables and recommends short stubs.

SICK describes the trunk + termination concept and the relevant network structure for CANopen installation.

5.2 Termination (critical)

Install exactly two 120 Ω terminations: one at each end of the trunk.

Never terminate stubs.

SICK installation guidance uses termination at ends of the trunk; CiA defines trunk/bus cable as terminated at both ends and stub as unterminated.

5.3 Cable, shielding, grounding (EMC robustness)

Use CANopen‑conform cable (twisted pair, appropriate impedance class).

Maintain a continuous shield strategy per machine EMC concept and ensure solid equipotential bonding/grounding practice.

CiA 303‑1 covers CAN ground and physical layer design recommendations; SICK manual emphasizes proper installation practices.

6) Bitrate selection: robustness vs. speed (125 / 250 / 500 / 1000 kbit/s)

CiA CANopen CC reference values (illustrative design limits):

• 1 Mbit/s: bus 25 m, max stub 1.5 m, accumulated stubs 7.5 m
• 500 kbit/s: bus 100 m, max stub 5.5 m, accumulated stubs 27.5 m
• 250 kbit/s: bus 250 m, max stub 11 m, accumulated stubs 55 m
• 125 kbit/s: bus 500 m, max stub 22 m, accumulated stubs 110 m

SICK AHM36 device-specific stub limits:

• 1 Mbit/s: single stub ≤ 1 m, sum of stubs ≤ 5 m
• 500 kbit/s: single stub ≤ 5 m, sum of stubs ≤ 25 m
• 250 kbit/s: single stub ≤ 10 m, sum of stubs ≤ 50 m
• 125 kbit/s: single stub ≤ 20 m, sum of stubs ≤ 100 m

Engineering rule: Apply the more conservative limit (typically the SICK table) for real installations, because it embeds device-/connector-/EMC margin.

7) Example network assessment

• Trunk total 20 m
• After 10 m: T‑connector with 5 m stub to AHM36 #1
• After another 10 m: AHM36 #2 at trunk end with 120 Ω termination
• Master end also 120 Ω termination
• CANopen‑conform cable, shield / grounding consistent

Assessment:

At 500 kbit/s:

• Trunk 20 m ≪ CiA bus 100 m
• Stub 5 m meets SICK’s 5 m max and is below CiA 5.5 m
• Cumulative stubs ~5 m ≪ SICK 25 m / CiA 27.5 m
  ​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​→ 500 kbit/s is technically compliant and expected to be robust if termination and EMC are executed correctly.

At 1 Mbit/s:

• Stub 5 m violates SICK (≤1 m) and CiA (≤1.5 m) stub limits
  ​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​→ 1 Mbit/s not recommended for this physical layout.

8) Field “Do / Don’t” (quick troubleshooting prevention)

DO

• Line topology, two terminations at trunk ends, short stubs, shielded cables (CANopen-conform), well grounding
• Select baud rate based on both trunk length and stub limits, if unsure, prefer lower baud rate for more robustness.

DON’T

• Don’t build a star network with long branches.
• Don’t add extra terminators or terminate stubs.