Re: Line impedance measuring & matching -- techniques

Thanks John for your reply.

>> I’m not sure I understand your question.
I mean to say that if CAN network is having problem in communication, and i
need to investigate what is causing mismatching of impedance. Then for that
:--
1> I will have to measure the line impedance in loaded & unloaded condition
2> Then check what is causing impedance mismatch.
3> correct these factors.

So how to measure line impedance in loaded & unloaded condition ?
Also How to correct this ?

Regards,
Dinesh

On Wed, Feb 3, 2016 at 12:40 PM, John Dammeyer <[email protected]>
wrote:

> I’m not sure I understand your question.
> As an example.  RG-58U coax cable has a 50 Ohm impedance.  If you attach
> that cable to a TV with a 75 Ohm output impedance and an antenna rated at
> 50 ohms you will have a mismatch and some of the signal value will be lost
> or reflections will affect the signal quality.
>
> Are you asking how cable is characterized to be 50 ohms?
>
> If you purchase Beldon Thick DeviceNet cable the blue and white wires have
> a characteristic impedance of 120 ohms at the frequencies that show up with
> CAN messaging.  To prevent reflections the ends of the transmission line
> need to be terminated with that same value of 120 Ohms.
>
> The CAN bus drivers are designed to drive a load.  That load is 60 Ohms in
> parallel with 120 devices that each have an input impedance high enough to
> not exceed the driver.
>
> For example.  Say your driver has a max output current of 70mA.  A CAN_H
> signal level of 4.5V would drive current out to the two 120 ohm resistors
> and also into each CAN receiver.  Most of the current enters the CAN_L pin
> which is held at about 1.0V.  So the voltage across the bus at worst is 3.5V
>
> If the receiver has an input resistance of 100K then the parallel
> combination of 120 resistors, each 100K in value, is 833 ohms.  That’s in
> parallel with the two 120 Ohm bus termination resistors.  Total resistance
> seen by the driver during a dominant is then 52 Ohms or 67 mA which is
> within specification of the driver.  That’s the DC side.
>
> The AC side is also interesting.  The cable design is 120 Ohms impedance.
> With 120 ohm resistors at each end the transmission line is matched and all
> the AC energy is absorbed by the resistors.  But if this were a 45 Ohm
> impedance cable you now have a mismatch and you get reflections which could
> mean the signal is degraded or bits are even changed.
>
> So you think, well I’ll terminate the cable with 45 Ohm resistors to once
> again correctly terminate the transmission line.  The parallel combination
> of those two resistors now means the drivers have to supply 156mA without
> even considering the parallel combination of the receivers.  That’s too
> high.
>
> What if you ran 300 ohm twinax cable like the stuff used for FM antennas.
>   Well first of all the CAN driver isn’t a balanced driver so that’s not an
> ideal cable even if you terminate the ends with 300 ohm resistors.  The
> lack of twists means the signal is susceptible to both magnetic and
> electrical interference.
>
> So could you run CAN with 300 ohm cable?  Of course you could.  Might even
> work really well.    I’ve run 1Mbps CAN between two ends of 100’ of Z
> wire.  That’s the pink 4 conductor unshielded telephone cable.  It worked.
> But that’s two nodes and a block of messages from one end to the other.  No
> idea if some parts of the cable had  ‘null’ spots where the signals might
> totally cancel out.  Not even sure what Z wire impedance is.  But I
> wouldn’t want to fly in an airplane that used fly by wire control signals
> that way.
>
> John Dammeyer
>
>
>
>
>
>
>
> From: [email protected] [mailto:[email protected]] On
> Behalf Of Dinesh Guleria
> Sent: February-02-16 10:22 PM
> To: CANLIST
> Subject: [CANLIST] Line impedance measuring & matching -- techniques
>
> Hi ,
> Please can anyone on list suggest , how to do Line impedance measuring &
> matching. For RS485 & CAN bus cables ?
> Regards,
> Dinesh
>
> --
> Archives and useful links: http://groups.yahoo.com/group/CANbus
> Subscribe and unsubscribe at www.vector.com/canlist/
> Report any problems to <[email protected]>
>