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Re: CPCI term resistors
I agree with the issue you bring up about bus lengths being longer on
cpci than pci, but as I understand it, the cpci series term resistors
are an implementation of reflected wave switching. At an incident
signal edge, the voltage level out of the resistor is lower than the
target voltage until the reflection from the end of the line gets back
to the resistor. If reflected wave switching is not used, end
termination would be required, as in a resistance to gnd, power or both.
Cpci spec mentions "stub termination minimizes the effect of the stub on
each board to the PCI backplane" in para 3.1.3.
I believe it also provides some attempt at impedance matching for source
termination of a reflected wave signal.
"K. C. Sriram" wrote:
> PCI uses reflected wave switching and it does not need any termination
> resistors. The length of the PCI buses are usually small limited to the
> CPCI is meant to be used for the backplane and the lengths are extremely
> large. reflected wave switching cannot be used and needs the terminations on
> the bus.
> -----Original Message-----
> From: Ingraham, Andrew [mailto:Andrew.Ingraham@compaq.com]
> Sent: Tuesday, March 20, 2001 4:02 PM
> To: email@example.com
> Subject: RE: CPCI term resistors
> > The CPCI specification requires 10 ohm series termination resistor on the
> > PCI
> > lines. This minimize the effect of the stub on the backplane. Standard
> > PCI
> > does not require termination resistors. Why is this?
> Standard PCI as a motherboard-only bus came first. Then came the connectors
> and PCI cards. Standard PCI was designed without the resistors.
> CPCI came later, and I believe their inclusion, along with a different card
> connector, allowed more slots to be used with less difficulty. At the time,
> the typical bus had three, maybe four PCI slots, which was somewhat
> inadequate for some cost-effective industrial applications. CPCI allowed
> eight without a bridge.
> > I am not sure the exact
> > reasoning behind these termination resistors.
> They somewhat isolate the stub from the rest of the bus, resulting in less
> impedance discontinuity, or less capacitive load, to the main bus at each
> stub. There is some added delay "through" the resistor, which is weighed
> against the reduced bus delay waiting for the bus to settle.
> This is sometimes called distributed termination.
> > Lastly, what would be the effect
> > of not having them ;-).
> I would expect you'd see worse ringing and longer settling times, but YMMV.
Ted Firlit (719) 594-8138
Senior Design Engineer
UTMC Microelectronics Systems
4350 Centennial Blvd., MS 1004
Colorado Springs, CO 80907