RE: Transceiver FMS interface

The impact of using normal mode vs. listen-only is bigger than just sending
ACKs; your node will also be sending error flags which may interfere with
proper bus operation if your bit timing is off. So unlike Kees, I would
recommend against using normal mode on the vehicle side unless there is an
explicit need to participate on the bus and send data. 

You might even want to investigate designing a dedicated
'hardware-listen-only' circuit in which the TX of the controller is not
connected to the transceiver. AFAIK this should be possible depending on the
transceiver, I remember a discussion like this a while ago with Steve
Corrigan chiming in on which transceiver to use for this. You can search the
archive for those messages.

cheers,
Bram
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of fflinera
Sent: donderdag 17 januari 2013 10:04
To: [email protected]
Subject: Re: [CANLIST] Transceiver FMS interface

Hi, all.

Thanks for your answer.

As conclusion,
I think that I can use standard 5V transceivers (it is more safe according
to Mr Dammeyer answer) as PCA82C251 on both sides of the FMS device and we
can use listen mode or normal mode in the CAN module connected to the
vehicle CAN network. I think the difference between listen and normal mode
is only the ACK slot response.

Nevertheless, due to this conclusion, is difficult to understand which is
the advantage of using an FMS interface.

If you want to use, for example, an USB/CAN device, Which is the advantage
to connect it using a FMS interface in the middle instead of connecting it
directly to the J1939/CAN vehicle network?

The objective of the FMS interface is do not disturb the CAN vehicle
internal network. But if you disturb the network due to a short-circuit
between CANH and CANL (due to a shortcircuit in a driver or even due to a
short-circuit bad connection in the CANH_CANL wires), the effect over the
internal vehicle network is the same if it happens in the CAN module of the
CAN/USB device than if the problem appears in the FMS interface of the CAN
module connected to the internal vehicle network.


From this point of view, an FMS interface device is more a device invented
to cause an expense to the truck owner that a safety device to prevent
possible malfunctions of the truck ECU network.

I can be wrong but this is my first reflection.

Thanks for your help, again.

--- In [email protected], "John Dammyer"  wrote:
>
> Hi,
> 
> There are some driver issues that must be addressed.  If you are hot
plugging your CAN nodes you have to ensure that the ground pin makes contact
first.   What we found is that if the supply voltage is 24V-28V and the
Power, CAN_H and CAN_L connections are made first there is a possibility
just before the ground finally connects the CAN bus could see a momentary
common mode potential of the 24V-28V power. 
> 
> For the 5V based CAN drivers that have a 42V maximum CAN bus voltage
that's not an issue.
> 
> For the 3V CAN drivers that have a 16V maximum it can be.
> 
> Over the period of a year I think I went through more than 10 drivers on a
number of  USB to CAN dongles that used a 3V CAN driver because we were hot
plugging (for testing and configuring) with connectors that could make in
any order.
> 
> OTOH, we hot plugged over 2000 nodes into 24V without issue since they had
5V CAN drivers.  And unfortunately we also lost almost 50 nodes in a
heartbeat when power was applied to a cable that had developed a short
circuit between the 48V rail (sitting at 56V charging the batteries) and
CAN_H.
> 
> John Dammeyer
> 
> -----Original Message-----
> From: canlist-owner@... [mailto:canlist-owner@...] On Behalf Of kees@...
> Sent: January-16-13 12:23 PM
> To: canlist@...
> Subject: Re: [CANLIST] Transceiver FMS interface
> 
> Hello
> 
> If you are going to connect anything to an existing CAN network, you can
allways kill the communication. This is independant of which transceiver
chip you are using. As long as uou make the right connections to CAN Hi, CAN
Lo and CAN Gnd and the existing bus is capable to drive one extra device,
there is no problem.
> 
> The ISO11898-5 standard only describes the "sleeping mode" when there is
no communication on the bus. So you can use your devices in a low-power
mode. It has nothing to do with extra protection.
> 
> As long as you are just listening on the bus, you will have no problem on
the J1939 network. Sending messages on the J1939 network is mostly not
appreciated by the truck manufacturers. If you put your CAN controller in
the listen-only mode, you will not be able to send any message, but if your
software is not sending anyway this not really needed.
> 
> The listen-only mode is normally only used as long as you are not sure
that all parameters are correctly. If you have some problem, like bitrate or
CAN Hi and CAN Lo crossed, with your device it will be going to send error
messages on the bus and so have influence on the bus. In the listen-only
mode this will not be the case. However if you see that you are receiving
messages and no errors (your Rx error counter = 0), there will be no problem
to switch to normal mode.
> 
> As long as you don't have to connect to the bus used in the connection
between truck and trailer (ISO11992) there is no physical difference between
the busses.
> 
> So I advise to use the same hardware for both ports. Possible problems are
mostly created in the software.
> 
> Hope this information helps.
> 
> Kees Zagers
> 
> 
> 
> fflinera schreef op 16.01.2013 18:03:
> > My objective is to design a FMS interface device.
> > I have planned to use a microcontroller with two CAN modules: one 
> > for the CAN/J1939 interface with the truck/bus network and the other 
> > for the CAN/FMS output.
> > My doubt is about which transceivers I must use in each part.
> >
> > For the CAN/FMS output, I suppose that a typical ISO11898-2 
> > compatible transceiver is OK (a PCA82C251 for example).
> >
> > But, for the CAN/J1939 interface, I think we need to use something 
> > special, because I must ensure that my device does not disturb the 
> > operation of the Truck/bus CAN network in case of failure.
> >
> > I think that these 2 options are possible:
> >
> > 1. To use an ISO11898-5 compatible transceiver like TJA1043 from NXP.
> > In this case, a bus failure can be detected and the FMS node can be 
> > disconnected of the internal bus by the microcontroller.
> >
> > 2. To use an standard ISO11898-2 compatible transceiver (PCA82C251), 
> > but the Microcontroller CAN module mus be programmed in "listen only"
> > mode. in this case, What would happen  if the transceiver causes an 
> > internal vehicle network failure?
> >
> > Please, Can anyone guide me whether any of these two options is 
> > correct or is there better solutions?
> >
> > Thanks in advance.
> >
> >
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