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Part Number: ISO1050
We use ISO1050 for CAN bus application got 30M @ 1Mbps question.
Please kindly let me know why R1 = 120 Ohm / R2 = 120 Ohm is not work. And R1 = 60 Ohm / R2 = Open is Work.
The test condition is transmissions 1000 times. CAN bus SPEC termination resistors is R1/R2 = 120 Ohm or not?
R1 = 120 Ohm / R2 = 120 Ohm @ 1Mbps test fail. CH1 and CH2 is CANH and CANL
R1 = 100 Ohm / R2 = 100 Ohm @ 1Mbps test fail. CH1 is ISO1050 Rx. CH3 and CH2 is CANH and CANL.
R1 = 60 Ohm / R2 = Open @ 1Mbps test Pass. CH1 is ISO1050 Rx. CH3 and CH2 is CANH and CANL.
Hi AJ Cheng,
Thanks for using TI E2E forums.
Yes the CAN standard (ISO11898-2) spec is for an effective value of 60 ohms ( = two 120 ohm end-termination resistors ). Can you please elaborate on the pass/fail criteria further ? I understand you are sending the data 1000 times Are you seeing any bit errors reported? Or any specific criteria (time spec etc.) which is listed as a fail ? The reason I ask this is because the amplitude levels in the scope shots look OK to me.
In Figure 1: (Scope shot): R1 = R2 = 120ohms; in dominant CANH = ~3.4V, CANL = 1.2V and Vod = CANH - CANL = 2.2V; this is typical and healthy swing for a dominant bit. In recessive state, both CANH and CANL should be similar and they are around 2.4V. Again, the amplitude levels are all within spec, so I am trying to better understand the fail classification.
Scope shot 2: R1 = 100ohm, R2 = 100ohm; Again I see healthy amplitude levels for both dominant and recessive states. Further, the RX output is faithfully following the CAN bus and is correctly reporting Recessive (RX = high; CANH - CANL = ~0) and Dominant (RX = low; CANH - CANL > 1.5V). so, just like above, any info here on why this is classified as a fail ?
Scope shot 3: R = 60ohms; R2= open; this is also similar to above. Signal level is OK for both states (Dominant and Recessive) and RX output is following what it sees on the CAN bus.
Below are a few things to double-check, this will help debug and understand the problem:
1) Does your USBCAN-2E-U, already has built-in termination ? I looked this up online, some vendors ship with built in termination and others have external termination - depending on the model and vendor ( I versus II) .
2) What is the capacitance (if any) connected to the CANH, CANL pins? This can be in the form of actual caps, or also through TVS diodes and/or chokes. A side question related to this is if the test passes at slower speeds, say 500kbps ? (for debug)
3) What is the data payload you are sending ? I assume you are sending this via the USBCAN? I see 5 dominant bits followed by 1 recessive bit. ( this usually happens when you try to send a string of same bit ). Can you try with a 50% duty cycle just for debug?
4) What is the TX input of the ISO1050 connected to?
Please let us know and we'd be happy to follow-up with you when we are back in the office on Monday
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In reply to Abhi Aarey:
Thank you for your reply. Update on below for your question.
1. I think our USBCAN-2E-U module not built-in termination. Because this module has two additional 120 Ohm resistors.
2. USBCAN-2E-U setting to slower speed mode is not work. I think is slower speeds problem.
3. I can not change duty cycle from USBCAN-2E-U and please kindly reference the test results from USBCAN-2E-U tool.
4. Please reference schematic on below.
In reply to AJ Cheng:
Thank you for your reply. Please kindly reference on below. Thanks for kindly help~
2. 57.9 Ohm
3. ISO1050 side
4. We have already removed B107 / B108 and D44.
Good day. Can I ask another question about ISO1050 EVM schematic? Why is the EVM put 60 Ohm termination resistor. Thanks~
In a real CAN network, we expect the two end CAN nodes to be terminated with 120 ohm resistor each to match the cable impedance. Two 120 ohms in parallel will give us an effective impedance of 60ohms.
On an EVM , since we have only one node ( = one transceiver), we directly use the effective impedance of 60ohms as a standard load.
This is also consistent with the CAN standard and CAN datasheets, where 60 ohms is used a standard value when measuring just one CAN transceiver.
I hope this helps.
Good day. We provide video record on below for you reference. Is it likely to be a matter of compatibility problem? If any suggestion please kindly let me know. Thanks~
R1 = 120 Ohm / R2 = 120 : 0:03 second is start transmission and 0:19 second got fail.
R1 = 100 Ohm / R2 = 100 : 0:04 second is start transmission and 0:06 second got fail.
Thank you posting this. On Friday and today, we have been in email and phone discussions with our Field team there to look at this. I am re-posting some of that here for convenience.
There is a possibility of the USB2CAN module doing something here, so compatibility could indeed be a factor. We saw the videos and in the beginning, the scope shot looks OK, but only later do we see the bits getting affected and moving around. This may be due to interference, or even the software trying to do something in the middle of transmission. **if** this was a hardware issue, we would have seen ***all ** bits getting affected and not only some in the middle.
Therefore, for simplicity of debug, we recommend trying this with two ISO1050 EVMs. We tested and reconfirmed this in our lab - two ISO1050s with one 120ohm resistor at each end were able to communicate without any issues. This was also verified in our lab today by Dan Kisling (thanks Dan). I am attaching the scopeshot here:
When testing with two EVMs, it is good to start with a smaller cable length (5m, 15m etc.) and also with slower speeds (250kbps, 500kbps etc). After these tests work smoothly, you can scale it to 1Mbps, 30m.
Good day. In this case if only R1 = 60 Ohm / R2 = Open can fixed this issue. Can we use this method?
If possible, please kindly help me to confirm if layout needs improvement. Thanks for help~
CANH and CANL
Tx and Rx
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