Common mistakes — bus extender

General CanCan problems (LED codes, box invisible on the bus, firmware update) are in Common mistakes. This chapter is what goes wrong when two segments are joined.

Devices on the far segment do not appear at all

Likely cause	How to tell	Fix
A segment is not terminated on its own	unplug the extender, measure each segment separately: it must read 60 Ω. One segment reading ~120 Ω has only one resistor	120 Ω at each end of each segment — four on the boat
CAN 2 not wired, or wired to a dead segment	nothing on that side is powered	connect it to a live segment and power-cycle the box once
One segment is at the wrong speed	a segment built for other equipment	both sides of this box are 500 kbit/s MUXEN, fixed. Nothing else works on it
The far segment has no power of its own	the extender powers no other device	the segment's devices are powered from the boat, not through the CAN cable

Note what is not on this list: the extender starts repeating as soon as it boots, before it has been given any instance number. A box still at the factory default relays traffic normally.

A brand-new box on the far segment cannot be addressed

Almost always: the extender itself has not been addressed yet.

While its own instance number is still the factory default, the extender occupies the parking address that every unconfigured device answers on, and it answers configuration traffic sent there itself, locally, without passing it across. The new box never hears the request.

Give the extender its two instance numbers first (Commissioning, step 3), then address the far-side box. The same rule as always still applies afterwards: one unconfigured box on the bus at a time, counting both segments as one bus.

Devices flicker in and out, values jump, alarms clear themselves

Likely cause	How to tell	Fix
Two devices share a function and instance, one per segment	muxen-uds -i can0 scan with the extender running lists the pair twice	renumber one of them. Instance numbers are unique across both segments, not per segment
Half-terminated segments	works on the bench, fails with the real cable lengths	see above — four resistors
The pair is overloaded	see below	reduce traffic, or do not split the bus this way

A segment that was commissioned on its own, from an instance list covering only half the boat, is the usual source of the collision.

Frames go missing under load, with no bus errors anywhere

Every frame crosses, so each segment carries its own traffic plus all of the other's. Two half-loaded segments make one full bus: the extender buys cable length and fault isolation, never bandwidth.

When more arrives than the far segment can carry, the box holds a short queue per direction and then drops the excess. It is dropped silently — no error frame, no counter, nothing to see on the wire.

Size the installation as one bus of the combined traffic. If the two halves together would overload a single 500 kbit/s MUXEN bus, joining them with an extender does not help.

Everything froze, or a frame seems to repeat for ever

Two extenders between the same pair of segments, or segments wired into a ring, make a frame circulate endlessly. Nothing in the box detects this.

One extender between two segments. Never a loop. If the boat needs three segments, chain them — A to B, B to C — never close A back to C.

I changed the instance number and nothing changed

The diagnostic addresses are built when the box starts. Write instance numbers with --reboot, or power-cycle the box afterwards.

Nothing shows up on device/31/<n>/…

The extender's own periodic status frame is on the CAN bus, but the Brain does not currently turn it into an MQTT topic. What you will see is device/31/<n>/boot at startup and device/31/<n>/uid on a scan, plus the box's generic I/O on device/17/<n>/life once EnGenIo has been enabled.

To check the box itself, read it directly:

muxen-uds -i can0 -F 31 -I <n> readconfig

Can I put NMEA 2000, J1939 or Scheiber equipment on CAN 2?

No. Both sides of this product are MUXEN at 500 kbit/s, fixed in the firmware. Other networks have their own CanCan applications — see the catalogue in CanCan gateways — Overview.

Can I update the firmware of a device on the far segment?

Yes, from the Brain, exactly as if it were on the local segment: the transfer crosses like everything else. Update the far-side devices before you touch the extender itself — while the extender is restarting, the far segment is disconnected from the boat.