CAN Bus Interface Question

something is not right in this schematic, did you copy/paste from some were or did you draw it.? I'm looking at 2resistors in parallel on the L/H can buss and that doesn't seem to be correct.
<edit>
But in any instance, tomorrow I will mesure the impedance on the CAN buss pins 2 and 7 and will pass along the info. For what I can understand, these terminator are in the inside of the OBDII connector/ buss lines and are NOT necessary to implement in any product to be connected to it.
As an Information quote and by experience, to the CAN buss we can only connect a single external device at the time for accurate readings.

<edit><March 27 14hs>
Did the mesurement over the pins2 and 7 and the impedance is about 40 Ohms, so it is confirmed again, these resistors are built in.

 

Send a email and clarify the issue. Also you need to know what kind of resistor. It is not enough to know 120 Ohm. OK I have leaned a few things in the past few weeks trying to find the parts to do my 12 volt mod on my CAN View. All I know is that the devil is in the details, it is true now and it has all ways been true. Don't screw up. Tight sphincters out there.

 

The schematic is not wrong.
Toyota Tech Manual says there are 120 ohm resistors on both ends, battery ECU and engine ECU.
You do not need extra 120 ohm resistor for CAN232.
There are a lot of CAN232 users in Japan and they are woking well without extra resistor.

Ken@Japan

 

Ken would that mean that my CAN- OBEII splitter works because the termination is built in? Several have questioned and warned me about using the "splitter" to hook up to the OBEII connector to run two devises. So far I can run the CAN-View and the ScanGage or the THHS together. I have not tried, as yet the OBEII modified CAN-View and the ScanGage together. Both of these pol the bus for OBEII information. I have been hesitating, but it is clear that other than the intake air temp there is no overlapping. Just want a second opinion before I try it out.

 

As you know, I'm using CANview V2 to. Not to long I added an ELMSCAN5 connected to my carputer for constant monitoring and data retrival/tracking and both devices conflict to each other because of the high speed in there shared communication buss.

 

I can't comment about the CAN-OBE II splitter because I know nothing about it.
Also, I don't know where your built-in termination is located.

Again, Prius has two 120 ohm termination on the front-end(engine ECU) and the rear-end(battery ECU), therefore no additional termination is required.

Some Japanese folks connect two CAN232's without any errors.

Ken@Japan

 

Opening it up reveals a very plainly burnt circut board. (In fairness, the CAN chip's probably ok, well protected, but the DB9/CAN side of the board itself is charred toast).

Hmm... let's go back and reread that whole 'protected' section. instead the power supply will be short circuited .

Ahhh... umm... Oops. Let's see, the thing was designed for approx 100mA. Power supply... yup, rated at 3000mA, output 14V... Short out the power supply, do some math, carry the one... Yup, I just sent nearly 42 Watts though that little circuit trace - poor thing never stood a chance. It didn't quite light up like a 40 Watt lightbulb, but briefly, it was close.

Seriously... take my soldering iron away before I kill again!



-Ken

[snapback]231550[/snapback]​

[/quote]

RIP for the CAN232

 

I posted details, it really is just a splitter of the cable. One plug in for the OBE II in the car two out. One to one connections of cables to both the output plug ins. Nothing complicated, apparently I am not the first to have this idea. It does work.

 

<div class='quotetop'>QUOTE(kdmorse @ Mar 29 2006, 06:19 AM) [snapback]231550[/snapback]</div>

I did once reversing the power.

Connected my CAN232 powered from DLC3 pins, but it did not work.
Found reversing the power, then turned pale.

Checked carefully, found a 7.5A DLC3 power fuse was gone.
Replaced the fuse, correct the power line, then my CAN232 started working again.
I was a lucky man.

Ken@Japan

 

We overlapped replies. Sorry about the dongle. The small skills I ever had are slipping... my hands were shaking as I did Daveinolys mod. It was OK with mine but someone elses. Norm told me to put jumpers across the SCART connectors and I chickened out and put them on the cable. Less chance of trouble!

 

If you got the polarity right, it shouldn't matter how much output
current your power supply is rated for. The can232 dongle will
draw exactly as much as it needs. Apparently, when power is applied
to the wrong pins, it suddenly thinks it needs a lot more. I would
venture a guess that this is simply from an inverse diode across the
power leads, to make sure that some fuse blows rather than sending
reverse polarity to all the chips on board. If there's no such
protection, then hopefully you just fried a regulator or something.
The makers of such dongles probably assume that you're always going
to plug them into a correctly-wired car, which always has power on
the right DLC pins...
.
_H*

 

To answer the question about the resistors, the CAN is designed in a bus topology. A resistor is required on both ends. From a strictly DC side, only one resistor is needed to 'pull down' the voltage across the data lines to 0 differential volts. However, when the data stream is at a high enough rate, its characteristics is more like radio wave AC. Resistors in this instance is used on both ends to counteract reflective waves which could cause false data. Ideally one would put a resistor at each endpoint, but then you'd mess up the DC components of the data, since the resistive equivalent of resistors in parallel become smaller and smaller.
Since your leads are somewhat long, and you split it, ideally it should be terminated somehow, but if it works, don't worry about it. Be sure to use twisted cable though if your leads are lengthy.