paul2@cs.exeter.ac.uk (Paul Day) (08/03/90)
I asked for advice on memory expansions and 51/4" drives -- several people sent me e-mail on the subject, and so here it is .... (n.b. all extracts contained in this posting were private e-mail, but all the people concerned gave their permission to re-post) MEMORY EXPANSIONS i) I think a public domain design for memory expansion was available from trantor.umd.edu via anonymous FTP. ii) The current issue of Amazing Computing, contains a schematic for a 512k board that connects to the expansion slot. (trapdoor) iii) The 'trapdoor' slot on the A500 will only decode for 1M of RAM. There are quite a few RAM expansions becoming available for the Amiga 500 which allow you to plug megabytes of memory into that slot (in most cases up to 4MB, but one particular memory expansion allows you to plug 6MB into that slot, most commonly called the 'A501 slot'), but they all have wire (oops -- scratch 'wire') a set of wires which go to a daughterboard that plugs into the GARY slot, which the GARY chip plugs into. Thus, the slot itself will only decode 1M of RAM, but there are memory expansions available which provide multiple megabytes of RAM by plugging into both this slot and the GARY chip socket, where they get the extra decoding for more memory. 51/4" DRIVES i) I use a 720k 5.25" 80 track ds drive on the Amiga 500 all the time. It was designed to use this exact drive. You should not use the Amiga power supply as it is a big load to carry. There are a few chips required to interface the two together. These are mainly required for autoconfig purposes. I don't have the circuit here with me. I got it from a developer kit or something that one of my friend had. I used vero board to glue it all together. I think the chips are a 7400 and a 7474. The circuit is also in the Amiga System Programers book by Abacus. Go to the book store and have a look at it. By the way, its a good setup once it is going. You can read/write IBM 720k disks directly on the Amiga with the MSH messy-dos file system or just backup or use 880k AmigaDos disks for much cheaper than the normal 3.5" stuff. It's also a great way to get huge volumes of NEWS from our 386 Unix box. No more long modem calls to work. Great stuff... ii) It's pretty simple just one or maybe two chips from memory. I don't have the circuit here at present, but it was floating around on PD disks here a year or so ago. I'll have to see if anyone's still got it. I interfaced my 5.25" drive by pulling apart an external 3.5" drive and using the cable & interface from that. (I sold the drive mechanism to a 2000 owner to use as an internal drive as 2000 internals are more expensive than 500 externals for some inexplicable reason). It makes xfering data from peecees a real cinch, plus it is great for backing up the hard disk as the 5.25" floppies are so cheap. ---- hope the information given here is of some use to someone, we found it useful. Paul ================ Putting a spoke in the ear of the unguarded ================== -- Paul Day, Centre for Connection Science, | VOICE (0392) 264066 Computer Science Dept, University of | JANET paul2@exeter.cs Exeter, Exeter, Devon, England. | UUCP paul2@expya.uucp ================ BITNET:paul2%cs.exeter.ac.uk.@UKACRL =================