CAN transciever ISO1050 EVM issue

Sneha,

In general, if a node is able to successfully transmit, the receive circuit of that node is implicitly validated as well. In a two-node CAN network, bear in mind that the (sole) receiver must be able to successfully send the ACK signal to the transmitter, in order for the transmission to be completed successfully. For information on how to properly terminate a CAN bus, please download SLLA270 from the TI website and look at figure 8. Regarding your question why a termination resistor is needed, I am reproducing below some good information I gathered from the CANLIST archives: (Below is a contribution from John Dammeyer, edited by me for brevity)

 "Although CAN isn't a truly differential signal, since it's referenced to ground, the principles are exactly the same. A signal change somewhere on the bus propagates in both directions. When it reaches the end of the bus if there isn't any termination or in other words if it hits a really high impedance it reflects. Like throwing something at a wall. It bounces.  

 In a nutshell the resistors do two things. They are placed at each end of a transmission line to match the impedance of the line (the AC side). They also serve as a load for the drivers (the DC side).

 Think of it this way. When the nodes are recessive, the CAN_H and CAN_L lines are brought to 2.5V relative to ground. There's very little difference in that voltage so almost no current through the resistors and therefore no voltage difference across.

 Now on a dominant, CAN_H gets pulled high to about 4.0V and CAN_L gets pulled low to about 1V resulting in a 3V difference and about 50mA. So where/how does this current flow?

 It comes from the transmitters 5V supply, out the CAN_H pin, flows down the CAN_H wire to the resistors and then back into the CAN_L line of the transmitter and to ground completing the circuit. Some of it also leaves the CAN_H pin, goes down the CAN_H wire and into the receiver CAN_H pin, through the 100k or so input resistance and out the CAN_L and ground returning both ways to the power supply of the transmitter. It does this for each receiver.

 Now if the terminations weren't there we'd only see the current through the high impedance inputs of the receivers.   That's a real candidate for electrical noise pickup. A 60R load needs a much higher radiated noise signal to influence the data bits.

 The termination should be at the ends of the bus (the AC side of things). In a perfect world, a properly terminated bus will have absorbed the energy of the signal pulses and not reflect anything back..."

Thanks for the elaboration Hareesh.

Now my next doubt is why a 49.9 ohm resistor is required between TXD and GND1 pin of ISO1050 EVM. 

As when I try to communicate by a board having this resistor to other not having it,transmission doesnot take place.But when the other board is kept first,successful transmission takes place.

So now for successful transmission and reception I am using boards:

1. One having 49.9 ohm resitor but no termination resistor.

2. Other having 60 ohm termination resitor but no 49.9 ohm resistor.

In short when I use TI ISO1050 EVM having both these resistors my communication fails.

Thanks

Sneha

Sneha,

The EVM was designed by a different business unit (i.e. the group that makes the transceivers). I'll see if I can contact someone in that group to understand why that resistor was put in place. It could be because of the fact that output of many T & M equipment are terminated into 50-ohms. I say this because I see reference to "SIGNAL GENERATOR" in Figure 4 of SLOU262. In any case, that resistor should NOT be used with c2000 devices. Our output buffers are rated for 4mA - 8mA. That 50-ohm resistor would present a load of 66 mA. Please remove that resistor and retry your experiment.

Hareesh

Hi Hareesh

Thanks alot.

I had already removed that resistor in the EVM board which I had prepared and only then was I able to establish successful CAN communication.I wanted to confirm my doubt.Now I can go ahead with the removal of this resistor from TI EVM board too,else its not working.

Thanks for your time.

Regards

Sneha