HALLAM@vax1.physics.oxford.ac.uk (03/09/89)
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.davidj@cernvax.UUCP (davidj) (03/13/89)
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