inuy@vax5.cit.cornell.edu (11/23/90)
Today I was looking through my hardware reference manual for my old Amiga A1000 and I got to looking at the external disk interface connector. This looks like a real easy place to build some neat add on's. The first thing that came to mind was a current track display with a pair of counters, but that requires parts. Not that that should stop one, but it does make it hard to 'play' on a day when every electronics store in the USA is closed. However after looking over the port a bit more I think I have hit upon a just about hardwareless useful hack! I have not attempted this yes as I need to get a pair of db23's (BTW anybody know where I can get db23's in small quanities?) Anyway, after looking I noaticed that track 0 line (TK0*, pin 15) and the disk write protect line (WPRO*, pin 14) are both open collector. I would be willing to bet that one could just unplug there external drive and plug in a db23 with pins 14 and 15 jumpered and that would make it very hard for a viri to write to track zero. What do you think? Can it really be this easy? Matthew
johnhlee@flute.cs.cornell.edu (John H. Lee) (11/29/90)
In article <1990Nov23.012503.1150@vax5.cit.cornell.edu> inuy@vax5.cit.cornell.edu writes: [...] >looking I noaticed that track 0 line (TK0*, pin 15) and the disk write protect >line (WPRO*, pin 14) are both open collector. I would be willing to bet that >one could just unplug there external drive and plug in a db23 with pins 14 and >15 jumpered and that would make it very hard for a viri to write to track zero. >What do you think? Can it really be this easy? > >Matthew Close, but there are problems. First off, simply jumpering those two together will: 1) cause TK0* to be active whenever a write-protected floppy is inserted (so the drive appears to be always at track 0), and 2) cause *all* drives to be non-track-0 writeable. You can fix these two problems by using a open-collector OR-gate to neg-logic AND the TK0* and SEL* signals together to drive WPRO*. But a more serious problem has to do with the fact that the signal is output- only from the drive and only a data-bit to the controller. You're not really write-protecting the drive, but merely telling the controller that the disk is protected whenever the head is at track 0. For the Amiga, I believe the floppy controller does not enforce write- protection. Software and the drive itself does. Also, the trackdisk device may not check the write-protect signal except when a disk is inserted (not unreasonable since it's difficult for the user to (de)write-protect a disk loaded in the drive.) In this case, a write-enabled disk will: (a) always appear write-protected (head at track 0 when disk is checked), or (b) will never appear write-protected (head not at track 0.) In the latter case, the trackdisk device will write on track 0 anyways despite the circuit. Then how to do what you want? One possible method is to disable the write gate signal to the drive whenever TK0* and SEL* are active. Possibly using one quad OC-NAND chip. But there are timing considerations (like making sure the signal is off at the right time w/respect to the drive select.) To sum it all up: I wish it were that simple. ------------------------------------------------------------------------------- The DiskDoctor threatens the crew! Next time on AmigaDos: The Next Generation. John Lee Internet: johnhlee@cs.cornell.edu The above opinions of those of the user, and not of this machine.