Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!aramis.rutgers.edu!paul.rutgers.edu!jac From: jac@paul.rutgers.edu (Jonathan A. Chandross) Newsgroups: comp.sys.apple Subject: Re: UDC problems Message-ID: Date: 25 Aug 89 20:37:15 GMT References: <8908220011.aa16711@SMOKE.BRL.MIL> <7500@microsoft.UUCP> Organization: Rutgers Univ., New Brunswick, N.J. Lines: 31 brianw@microsoft.UUCP (Brian Willoughby) describes his experience with the UDC: > It's that 1% that bothers me. Actually, I get 0% bahavior after my CPU > heats up. I tried replacing all my motherboard 74LS138 chips which > .... > and the UDC utilizes RDY for transferring disk sector info to the Apple. > Just hook a logic probe up to the slot bus and watch RDY. I don't see > how the NEW UDC (the old UDC was much different, and DID support the II+) > can work with a 6502 or 65C802 in Emulation mode (which the UDC forces > every time you use the drive). I don't have a UDC, but from your description I think I know what might be causing the problem. If I understand correctly, the UDC disables the on-board 6502 using RDY and uses DMA to store the result of a disk operation directly into memory. The problem might be slow motherboard DRAM. I've run into this problem on other computers. As the DRAM warms up, it's access speed slows down. If the speed was marginal before (ie the DRAM was just fast enough to work) you will start seeing some flakiness. Don't forget that you lose some memory bandwidth to the refresh circuitry. This couple with slow chips might just be enough to cause the strange behavior you cite. Jonathan A. Chandross Internet: jac@paul.rutgers.edu UUCP: rutgers!paul.rutgers.edu!jac