Path: utzoo!attcan!uunet!ingr!b11!xenon!kerl
From: kerl@xenon.UUCP (Dan Kerl)
Newsgroups: comp.arch
Subject: Re: Problems with NeXT (25 MHz NuBus clock discussion)
Keywords: NeXT, NuBus, P1196
Message-ID: <103@xenon.UUCP>
Date: 20 Oct 88 22:13:53 GMT
Organization: Intergraph Corp. Huntsville, AL
Lines: 28

According to the 17-October issue of EET, the NuBus bandwidth of the NeXT
system is 25 Megabytes per second, not that NeXT's implementation of NuBus
has a 25 Megahertz clock. This bandwidth is consistent with the NuBus clock
rate of 10 MHz, since it is a 32-bit bus.

With a 10 Mhz clock, the data rates on NuBus are as follows:

	word burst length	rate (in Megabytes/sec)
------------------------------------------------------------------------------
	1 (4 bytes)		13 1/3 (competing with another master)
	1 (4 bytes)		20     (parked on bus - no competition)
	2 (8 bytes)		26 2/3
	4 (16 bytes)		32
	8 (32 bytes)		35 5/9
       16 (64 bytes)		37 11/17
------------------------------------------------------------------------------
Note that the arbitration time for subsequent masters of NuBus can be hidden
if the burst length is 2 words or longer. There are three bus cycles required
for arbitration; the first being a cycle in which the request line is
inactive, followed by 2 arbitration cycles.

With respect to the Mac II implementation of NuBus, the biggest problem I
have with it is their lack of support for bus retries (ACK=try again later)
I don't know if NeXT has this problem or not...

Dan Kerl
Intergraph Corporation
uunet.ingr.b11.xenon.kerl