Path: utzoo!attcan!uunet!cbmvax!daveh From: daveh@cbmvax.UUCP (Dave Haynie) Newsgroups: comp.sys.amiga Subject: Re: Lucas Accelerator Message-ID: <5826@cbmvax.UUCP> Date: 26 Jan 89 23:06:20 GMT References: <819@faui10.informatik.uni-erlangen.de> Organization: Commodore Technology, West Chester, PA Lines: 39 in article <819@faui10.informatik.uni-erlangen.de>, mlelstv@faui45.informatik.uni-erlangen.de (Michael van Elst ) says: > You don't have an 68881 ? ... > The solution is to connect the ChipSelect with DSACK1 on the 68881. > The cycle will terminate and result in an exeception that tells > the OS that no REAL 68881 is there. That's won't work. Such a setup would just result in a coprocessor violation, since the 68020 would think a 68881 was there, but the data would be garbage. To run a 68020 without a 68881 and get the F-line exception generated properly, it's necessary to run the 68881 chip select to the 68020's bus error (BERR*) input. On the Amiga, you'll also have to make darn sure that the bus error signal going to the 68020 isn't the same line as the bus error going to the expansion bus, since on the expansion bus, the BERR* line is used to indicate a PIC collision, which makes everything immediately get off the bus. The way to wire this up is as such: |\ BERR* from Amiga ------| |------------------+----- BERR* to '020 |/ 7407 | |\ | 68881 CS* ------X X--------------| |--------+ jumper |/ 7407 The jumper can be closed when there's no 68881. A more elegant solution would gate the 68881 CS* with the 68881 SENSE* output, so the BERR* line is automatically switched in when the '881 isn't there. > Michael van Elst > > E-mail: UUCP: ...uunet!unido!fauern!faui45!mlelstv -- Dave Haynie "The 32 Bit Guy" Commodore-Amiga "The Crew That Never Rests" {uunet|pyramid|rutgers}!cbmvax!daveh PLINK: D-DAVE H BIX: hazy Amiga -- It's not just a job, it's an obsession