OK, i'm strongly adverse to altering a MB except for adding sockets. That being said, I've designed a sandwich board to fit between the MB & the 65C22 VIA chip for internal drive device number selection. I'm just left with a couple of questions....
First, is there anything else connected to pins 15 & 16 other than the capacitor & ground? I've examined the schematics, and there doesn't appear to be. Visually inspecting the board also doesn't seem to show anything. But, thinking on it, that would mean that those pins aren't used for anything at all, other than power up device # selection.
Second. I noticed, while examining the schematics, that the ATN goes to the VIA chip after going through the 74LS chip which seems to invert the signal (from Lo to Hi from what I've read.) Would grounding this signal out (forcing it to Lo,) here also disable the floppy drive? Would include a resistor as well (of course.)
Post by feedingdragon on Jun 23, 2019 12:00:22 GMT
Been doing more research. Pin 17 VIA chip (PB7) should pose no problem grounding out. I'm not 100% sure about pin 40 of the VIA (CA1,) but indications are that it wouldn't be a problem. The problem I'm having difficulty in answering is how the gate array (MOS 251828-3,) chip will respond if pin 24 (ATNA) is grounded. Cannot find a datasheet on it. CA1 operates as the interrupt line, so just disconnecting it may work. But I'm again faced with what the gate array is doing. Don't really want to design & purchase a board until I know it's going to work.
Assuming nobody has any information telling me otherwise, I'm going to try installing the VIA chip with pins 40 & 17 disconnected. Should be safe, and will tell me if that would work or not.
For one thing you should NEVER use a 65C22 or R65C22 on ANY 8 bit Commodore computer. That's because these chips IRQB output is totem pole instead of open collector/open drain output as real MOS 6522 chips are.
For another, a CORRECT replacement is the currently produced W65C22N (NOT W65C22S) which you can order from Mouser - it is made by WDC, the Western Design Center. You can safely use these in the VIC-1540, 1541, 1571 disk drives and the good old VIC-20.
While the original MOS 6522 consumed about 60mA of power, Western Design Center's VIA consumes about 3mA when used at 1-2MHz as it is in Commodore 8 bit computers and drives.
Post by feedingdragon on Jun 24, 2019 21:14:25 GMT
I wasn't actually planning to replace the chip that is currently soldered onto the board. Just de-solder it and install a socket. Then place the sandwich board between. It's looking more and more like I'll need to just disconnect either pin 40 and/or pin 17 to block the ATN signal. Pin 40 operates as the interrupt line, (both generation & detection,) so that seems a likely choice.
All that being said... I may end up having to replace the chip, and that's good information to have
Managed to do some testing. Disconnecting Pin-17 of the VIA will make the drive invisible. The only issue is that the drive motor won't stop running. Going to have to try disconnecting pin-1 of U113 next, and finally settling for the major MB alteration as a last resort. Though, I may just put up with the motor's constant spinning. Or put in a 2 pole switch that also disable the power to the motor. Since I've already socketed the VIA chip, that is my first choice. Only need to get a single sandwich board for both disable the drive completely or change the device number. I'm already working to actually use dip switches instead of clunky toggle switches. Though, I may have to go with toggles anyway. Finding a way to panel mount dip switches without using a piece of Circuit board with a standoff Y screws. Though, that sounds eminently doable, and could actually come out looking rather nice Put the Floppy power & a bank of dip switches above the cart port, could look rather nice.