CAN on a 100 ohm or less bus

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Automation Artisans Inc.
http://www.autoartisans.com/ELS/
Ph. 1 250 544 4950

-----Original Message-----
From: canlist-owner@... [mailto:canlist-owner@...] On Behalf Of Michael J. Noone
Sent: Tuesday, August 25, 2009 10:32 AM
To: canlist@...
Subject: [CANLIST] CAN on a 100 ohm or less bus

Hi – we’re currently designing a system from the ground up that will be running a CAN bus. We are having custom cable made for it, and the necessary thickness of cable insulation (to maintain 120 ohm differential impedance) is making me very sad. My cable vendor suggested I try running a lower impedance bus to decrease the size of the twisted pair. My knee jerk reaction was to say no to this – but now that I’m thinking about it – it makes sense.

 

What should I be worried about with regards to doing this? I would of course terminate the bus with whatever impedance the cable was – not 120 ohms.

 

Also, note that every node that will be on this bus is being designed in house. So we are not worried about maintaining compatibility with other vendor’s parts.

 

Thanks,

 

Michael J. Noone
Boston Dynamics
Phone: 617-868-5600 x271

 

 

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And what John says.  But wait.  Could the great and wonderful John Dammeyer be wrong here – Does the TI datasheet actually list receiver input resistance.  Well low and behold, John sits corrected.

 

RID   Differential input resistance D at 0.7 VCC, Rs at 0 V             min  40      max  100  kW

RIN   Input resistance, (CANH or CANL) D at 0.7 VCC, Rs at 0 V  min  20      max   50   kW

 

Cheers,

Steve C.

 

From: canlist-owner@... [mailto:canlist-owner@...] On Behalf Of John Dammeyer
Sent: Tuesday, August 25, 2009 1:14 PM
To: canlist_NOT@...; canlist@...
Subject: RE: [CANLIST] CAN on a 100 ohm or less bus

 

It's not really an issue depending on your overall network configuration.

 

Here's why.  Two termination resistors in parallel make 50 ohms.  If the maximum Dominant voltage on CAN_H is 4.5V and CAN_L is min 0.5V then the driver will try and push 80 mA through this pair of resistors. (4.0/50).  With even two nodes that exceeds the drive of the device.  But that's worst case.

 

Typical is 3.5V to 0.5V or 3.0/50 = 60mA which is still a tad over the SN65HVD251 recommended max drive.  A set of 120 Ohm resistors does work.  However, it looks like the driver can sustain a short circuit of 200mA.

 

But, and here's the big but.  The other limit will be the number of nodes on the bus.  Althought the TI data sheet doesn't state it some of the other CAN drivers do.  That's the receiver input impedance.  A worst case value of 5K for example by itself isn't much but put 100 nodes in parallel and it becomes significant at an extra 500 ohms in parallel with your 50 Ohms.  That could raise the max current with the 3V differential to 66mA.  And if it tries to drive the CAN_H to 4.5V you end up with even more drive required.

 

It depends if you want to design for worst case.

 

In real life, using the above drivers and 120R termination, I found the dominant to recessive voltage to be about 2.75V with the ACK bit (all 50 nodes transmitting at the same time) rising to 4.5V

 

I can send you a photo if you'd like.

 

John Dammeyer

 

 

Automation Artisans Inc.
http://www.autoartisans.com/ELS/
Ph. 1 250 544 4950

-----Original Message-----
From: canlist-owner@... [mailto:canlist-owner@...] On Behalf Of Michael J. Noone
Sent: Tuesday, August 25, 2009 10:32 AM
To: canlist@...
Subject: [CANLIST] CAN on a 100 ohm or less bus

Hi – we’re currently designing a system from the ground up that will be running a CAN bus. We are having custom cable made for it, and the necessary thickness of cable insulation (to maintain 120 ohm differential impedance) is making me very sad. My cable vendor suggested I try running a lower impedance bus to decrease the size of the twisted pair. My knee jerk reaction was to say no to this – but now that I’m thinking about it – it makes sense.

 

What should I be worried about with regards to doing this? I would of course terminate the bus with whatever impedance the cable was – not 120 ohms.

 

Also, note that every node that will be on this bus is being designed in house. So we are not worried about maintaining compatibility with other vendor’s parts.

 

Thanks,

 

Michael J. Noone
Boston Dynamics
Phone: 617-868-5600 x271

 

 

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