Inmos links, the C004 and everything.

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2 total messages Started by HALLAM Wed, 08 Mar 1989 17:31

#2531

Author: HALLAM
Date: Wed, 08 Mar 1989 17:31

18 lines
1316 bytes

In respones to the recent letter from D.Bareham it is interseting to note
that he does not explain why the C004 slows down links.

The T.puter data sheet states that the T8 is capable of sending the acknowlege
about 5 bit periods after the start of receiving the message. This would
suggest that the C004 which introduces a 1.5 bit delay should not slow
the link. Unfortunately the link is not able to begin transmission of the
next byte until 5 bit cycles after receiving the acknowlege. 

Appologies to Dave Jefferson of CERN who worked out all this stuff for 
pre-empting but people seem to need to know !

Note to the guys at Inmos : Why didn't you tell us in the data sheet !
You could have figured that we would work it out sooner or later anyhow.

     P.Hallam-Baker, 

Oxford ZEUS collaboration.

Re: Inmos links, the C004 and everything.

#2548

Author: davidj
Date: Mon, 13 Mar 1989 07:38

23 lines
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We are preparing a paper on Inmos Links which will be presented at the
'Computing in HEP' conference at Oxford in April.  However to summarise
quickly....

The T8 does not send an ack until 5-6 bit periods after receiving the
start bit. And does not send the following datya byte until 5-6 bit
periods after receiving an ack. Thus the cable connecting two t8's
has to be very short unless you want to drop the data rate. The 5-6
figure arises because of jitter on the signal and clock sync at each end
of the link. There are additinal factors due to cable delayt, buffer delay,
c004 delay and which direction the wind was blowing when you made the
measurements. We have derived a formula which accurately predicts the
transfer rate between: t8-t8, t8-t4, t4-t8 and t4-t4 - see the proceedingd
of the conference for more details....

The c004 slows down the data transmission as it resync'c the data before
forwarding it, the delay is approx 1.9 bit periods at 20 Mbps link speed


	Dave Jeffery

CERN
Geneva

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