Bus health

CAN controllers keep score of every error they see. The Tercio FD reports those counters four times a second, so a bad cable or a missing terminator shows up as a number instead of a mystery.

In Tercio Control, open Tercio FD at the bottom of the side bar. In Python, read bus.adapter_status.

Bus state

Error active

HealthyNormal. Both error counters below 96.

Warning

Errors seenA counter reached 96. Frames still get through, but something is wrong.

Error passive

Error passiveA counter reached 128. The adapter stops signalling errors so it cannot disturb the bus.

Bus off

Bus offThe transmit counter passed 255. The adapter left the bus and tries to rejoin every 50 ms.

The drivers have the same counters: an S1 in error passive shows the CAN errors warning.

Counters

Field	Meaning
Transmit errors (TEC)	Rises by 8 for each failed transmission, falls by 1 for each successful one.
Receive errors (REC)	The same for reception.
Last error	The type of the most recent error, see below.
To the bus	Frames delivered to the bus (acknowledged by some node).
From the bus	Frames received from the bus.
Dropped out	Frames from the computer that could not be sent: the queue was full, or no node acknowledged them within 50 ms.
Dropped in	Frames from the bus lost because the computer did not read fast enough.
Bad USB frames	Damaged frames from the computer, rejected by their CRC.
Bus-off events	How often the adapter went bus-off.

Reset counters zeroes the traffic statistics; the error counters belong to the CAN controller and fall by themselves as frames get through.

Last error

Error	Usual cause
No acknowledge	Nothing else on the bus is powered or listening. Check that the S1 has power and that CAN-H and CAN-L are not swapped.
Stuff error, Form error	Noise or reflections: a missing or extra terminator, an untwisted cable, a long stub.
Bit 0 error, Bit 1 error	The adapter could not drive the bus to the level it sent: a short, a swapped pair, or a node on a different bit rate.
CRC error	Corrupted frames: noise, bad termination or a poor ground connection.
None / No change	Nothing new since the last report.

The 60 Ω test

With everything powered off, measure between CAN-H and CAN-L anywhere on the bus. About 60 Ω means two terminators, as it should be. 120 Ω means one is missing; 40 Ω or less means one too many.

The status frame

The adapter sends this on its own channel (ID 0x800) every 250 ms while the port is open, and in answer to GetStatus and ResetCounters. See Serial link.

Adapter status28 bytes

0bus stateu8

1tecu8

2recu8

3last erroru8

4to CANu32

8from CANu32

12dropped to CANu32

16dropped from CANu32

20framing errorsu32

24bus-off eventsu16

26reservedu16

Offset	Field	Type	Meaning
0	bus state	u8	0 error active, 1 warning, 2 error passive, 3 bus off
1	tec	u8	Transmit error counter
2	rec	u8	Receive error counter
3	last error	u8	Last protocol error code (0 none … 6 CRC, 7 no change)
4	to CAN	u32	Frames delivered to the bus
8	from CAN	u32	Frames received from the bus
12	dropped to CAN	u32	Queue full, or not deliverable within 50 ms
16	dropped from CAN	u32	The host did not read fast enough
20	framing errors	u32	Damaged serial frames from the host
24	bus-off events	u16	Times the adapter went bus-off
26	reserved	u16	0