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Re: Devices cannot drive and recieve signals at the same time?
- To: Mailing List Recipients <firstname.lastname@example.org>
- Subject: Re: Devices cannot drive and recieve signals at the same time?
- From: email@example.com (Wen-King Su)
- Date: Mon, 25 Jan 99 12:26:13 PST
- Delivered-To: pcisig@teleport.COM
- Resent-Date: Mon, 25 Jan 1999 12:58:33 -0800
- Resent-From: firstname.lastname@example.org
- Resent-Message-ID: <"z6xWV3.0.qy3.LBDhs"@electra.znyx.com>
- Resent-Sender: email@example.com
>I am trying to get a better understanding of the following statement:
>on page 112 on Rev 2.2 point # 9 the Spec states that " Devices cannot
>drive and recieve signals at the same time" and than it goes on to give
>the reason why " System timing analysis considers the worst signal
>propagation case to be when 1 device drives a signal and the signal
>settles at the input of all the other devices on the bus. In most cases,
>the signal will not settly at the driving device until some time after
>it has settled at all the other devices"
That statement refers to the fact that PCI bus signals are supposed to
be source-terminated. The worst case situation is not the case that you
described, but the case where the system is designed to contain a maximum
of two PCI devices that are separated by a great distance, such that
signal originating from one device reaches the other device barely within
the spec limit. The logic level at the driving end won't settle down to
a legal value until the signal reflection comes back, and that takes as
long as the forward signal propogation.