RE: Fwd: CAN HW compability

That’s because you are probably working on a fault-tolerant CAN network using FTCAN transceivers where the termination is distributed over the network. Each node has its own internal switchable termination (depending on the active/error state of the transceiver), with the values calculated so that the total termination value is around 100 ohm for the bus. Because the termination is switched, you cannot measure it when the nodes are unpowered (which is why you don’t see it).

 

If that is the case, you are using the wrong transceivers. You are probably knee-deep in TJA1054 territory.

 

Secondly, if you get the correct transceivers you still need to figure out the termination. Internal termination for FTCAN networks with a variable number of nodes is calculated in such a way that the nodes that are required (i.e.: always connected to the bus) have relatively low internal termination values so they contribute most of the overall bus resistance, while nodes that are optional have higher values so the impact of their absence on overall termination is limited. There are no general rules on how to distribute the termination values over the network, although they are generally lower for nodes on longer cable stubs. You want the bus to be as close as possible to 100 ohms, but 120 ohms (due to absent optional nodes) will do just fine. It would not surprise me that every OEM has their own “secret sauce” for FTCAN transceiver/termination requirements.

 

Thirdly, most transceivers have minimal termination values they can support, so an FT CAN network with only two nodes (the side of the parking assistance module) may require some creative tinkering to get it to work, especially if the other node has a relatively high termination due to the fact that it is an optional extra in the design.

 

It’s most likely going to be relatively easy to switch out the parking assist module for something of your own making: either pick a high-enough resistance value so your impact on the already balanced bus is negligible (I would venture 250 to 300 ohms times the total number of supported nodes in the network), or open up the module that you want to replace and figure it out using a magnifier and a good cup of coffee.

For the side of the parking assist module, pick the lowest resistance your transceiver can support, keep the bus as short as possible and hope for the best. Bus resistance will be far above the ideal 100 ohms, but for really short runs it may just work. Another solution might be that you add a separate (non-switched) terminator in the network, but all bets are off how it will respond – I would assume that FT operation is out of the question.

 

Disclaimer: there are a lot of assumptions in this message, and FTCAN can be a tricky beast to conquer. Don’t take my words for gospel ;-)

 

cheers,

Bram

 

From: [email protected] [mailto:[email protected]] On Behalf Of Job Race
Sent: vrijdag 8 mei 2015 9:14
To: [email protected]
Cc: [email protected]
Subject: Re: [CANLIST] Fwd: CAN HW compability

 

Hi Daniel

Thank you for your answer

BTW by "Sensors" i mean "Parking asistance module". sorry.

 

Adding termination was my first response but:

1. Adding any resistor between CANH & CANL smaller than 200kohm resulted in complete bus break down i.e. signal almost straight. as you increse the resistance the signal is shorter and shorter until it is gone. not to mention two 120 ohm.

2. The original entire network does not appear to have termination resistors. The way i know that is that before i disconnected the parking assistance module i disconnected car battery and tested resistance between CANH & CANL and it read about 1M ohm (double checked) but as you can see in picture 2 the signal looks very good

PS: the MCP2551 has basic (unpowered) 47kohm resistance between CANH & CANL

 

Best Regards

Job

 

 

2015-05-07 16:12 GMT+03:00 Daniel Perron <[email protected] <mailto:[email protected]> >:

Hi Job,

 

The first picture really show you that you are missing the terminator resistor.  High rise and a very slow drop! Also you have the ringing echo on the high rise side.

 

You need to add a terminator. What is the value you put there?

 

By definition  you need terminator on both ends.

Is this sensor at the end of the bus? There is a good chance it isn’t so you will need to add a terminator on the sensor also. (2 terminators needed!).

 

 

Daniel