sgt@alice.UUCP (08/08/87)
Recently, I bought from a friend a standard IBM-PC type 5.25" disk drive, case, and power supply to hook up to my Amiga. I dug out a description of the required adaptor that was posted by Randy Spencer (spencer@oberon.UUCP) on April 2, 1986. I was careful with the wire-wrapping, but now have the following two problems on which I seem to be stuck: The circuit, Randy claims, was reverse-engineered from an Amiga 3.5" external drive. This is obvious when I hook it up, as the system auto-mounts it as an 80-track drive, which it isn't. First Question: How to disable this auto-configuration, or how to change it so it returns the correct configuration information. (better yet - I'll wire in a switch) I'm fairly handy with ttl digital, so a good description of what signals have to get sent back in response to what stimuli would suffice. If anyone wants, I'll mail/post the resulting circuit when I get the info. Also, where can I get a description of the disk drive signals on the Amiga's disk connector. Neither the pinout section of the 'Intro to Amiga' manual nor the Hardware Reference Manual (AW version) has it. Some of the register bits look like they go directly out; but how for example does the software (driver?handler?Dos?) twiddle them during the auto-mounting/ My other problem comes when trying to test the drive even though it's improperly configured. I can't get the PC utilites to touch it while its mounted, nor can I unmount it nor remount it correctly. I let the standard format command have a crack at it anyway, knowing that when it got past track 39 I'd have to cancel it to keep from hurting the drive. It never gets this far; but rather always craps out at around tracks 18-25. This is with several different DS/DD disks that have been used on a PC sucessfully for a while. Format often retries successfuly at some tracks shortly before dying. Some hardware details: The drive is a Control-Data #77711805, 48tpi, double-sided full-height drive. I've got a total of about 30" of ribbon cable from the drive to the back of my external 3.5", including the interface board. Is that too much? I recall from my trs-80 days we were allowed 10 feet. I removed the terminator resistor pack from the drive. Before doing that, things were worse: It would only step towards track 0, and format wouldn't even verify track 0. Aparently the Amiga can't handle the current needed to pull down the line with a 150 ohm resistor, at least in the step-direction case. Does anyone have any ideas here? Must I get the drive aligned at some rediculous (sp) price? Is it related to the configuration info being wrong when it is mounted? (Like maybe the 3.5's get different write precompensation? I know it sometimes has to change on inner tracks) While I'm on that, could someone define 'write precompensation' as used relating to floppies, or suggest a good reference. I know that its measured as time (usec) and on some controllers is adjustable. Another request: Can someone point out a good reference on floppy disk technology, particularly the MFM recording scheme? I know roughly how the bits are recorded, most texts describe FM and MFM, but what I want to know is details like how the start of a sector is defined so the controller can find it. Given that information, I could probably figure out how to make the Amiga's disk interface read foreign MFM disks. I know MSdos disks have been done; but I want to try a filesystem handler for them, so we can dispense with special utilities and just use our same tools as if they were 'DOS ' type disks; most of the time only the AmigaDog guts need know the difference. Sorry for being long-winded. Please help me! Thanks much in advance... Steve Tell, Student: Duke University. Cheif Engineer: Cable 13 / Duke Union Community Television. Summer Employee: AT&T Bell Laboratories, Murray Hill, NJ. "Wouldn't you rather be interesting than nice?" from Cyrus X: The Movie. (A cable 13 production) -- Steve Tell, Student: Duke University. Cheif Engineer: Cable 13 / Duke Union Community Television. Summer Employee: AT&T Bell Laboratories, Murray Hill, NJ. "Wouldn't you rather be interesting than nice?" from Cyrus X: The Movie. (A cable 13 production)
grr@cbmvax.UUCP (George Robbins) (08/09/87)
In article <7165@alice.UUCP> sgt@alice.UUCP (Steve Tell) writes: > Recently, I bought from a friend a standard IBM-PC type 5.25" disk > drive, case, and power supply to hook up to my Amiga. I dug out > a description of the required adaptor that was posted by > Randy Spencer (spencer@oberon.UUCP) on April 2, 1986. > I was careful with the wire-wrapping, but now have the following > two problems on which I seem to be stuck: There are slight differences between the 5.25 adapter card and the 3.5" adapter card. There is a mechanism to return a serial drive type identifier over the RDY line. In the 3.5" this is all 0's, in the 5" drive it should be alternate 1's and 0's. > The circuit, Randy claims, was reverse-engineered from an Amiga 3.5" > external drive. There is little need to reverse engineer anything. Both the 3.5" and 5.25" adapters are included in the A1000 Schematics and Expansion Specifications document, available from Commodore for $20. > Also, where can I get a description of the disk drive signals on the Amiga's > disk connector. Same place... > I removed the terminator resistor pack from the drive. Before doing that, > things were worse: It would only step towards track 0, > and format wouldn't even verify track 0. Aparently the Amiga can't > handle the current needed to pull down the line with a 150 ohm resistor, > at least in the step-direction case. Right, the 3.5" floppies have permanantly installed 1K pullups on each drive, for a total of no more than 250 Ohms total. The A2020 drives are specified similarly. Some systems will work with 150 Ohm pullups, but a lower value is wise. One other thing to check is that your drive can reliably handle a 3mS step rate. There is some software way to adjust the step rate discussed a while back but I don't recollect the details. I'm not sure about the precomp issue, fixing the drive ID code will take care of that I'd expect. -- George Robbins - now working for, uucp: {ihnp4|seismo|rutgers}!cbmvax!grr but no way officially representing arpa: cbmvax!grr@seismo.css.GOV Commodore, Engineering Department fone: 215-431-9255 (only by moonlite)