Apple SmartPort Pin Directions

How can you deal with signals that might be inputs or might be outputs, in a way that’s safe and won’t damage sensitive electronics? In my last post I described three different types of Apple II family floppy drives. Since then, I’ve started looking further into SmartPort emulation for the Floppy Emu, which would extend 800K floppy and hard drive emulation support to the Apple IIc and IIgs. The required pinout on the D-SUB 19 pin connector is almost the same as what’s currently used by Floppy Emu, but not quite identical (inputs to the computer marked in bold):

Comparing the Floppy Emu pinout to the IIc and IIgs, a few pins must change from computer outputs to computer inputs. And a few other pins marked with * take on a new bidirectional behavior, being used both to control the floppy drive and to sense the type of the connected drive. All the non-power pins for Floppy Emu are connected to a CPLD, and can be switched from inputs to outputs with a firmware change, so by itself that’s not a problem. The trouble arises if you want to use a single Floppy Emu board on different computers at different times, functioning in different modes. If the Emu is configured for the wrong mode for the computer it’s presently attached to, it could damage both the Emu and the computer. Sooner or later somebody (or me) would make that mistake.

Let’s look at what this means for SmartPort emulation on an Apple IIc or IIgs. On the IIgs, pin 4 is no longer a ground pin, but instead is used to switch between Apple 3.5 drives and other types of drives. We’re not trying to emulate an Apple 3.5 drive yet, so we can safely ignore that difference and continue to use pin 4 as GND. On the IIc, pin 9 is an external interrupt. We don’t need that for SmartPort emulation, which is fortunate because it’s not even connected on the Floppy Emu end.

Pin 10 is the main problem. On the Mac, it’s an output from the computer, but on the whole Apple II line it’s an input. It either indicates the disk’s write-protect status (normal disks), or serves as an ACK signal (SmartPort). If the Floppy Emu is configured to output a WPROT or ACK signal on pin 10, and is connected to a Mac that outputs PWM on pin 10, the two chips will fight each other and cause damage. Zap!

Pin 16 is also a problem. On the Mac and the Apple II/II+/IIe, it’s either an output from the computer, or a +5V connection that will look like a constant logical 1 output. But a SmartPort device turns things around, and uses pin 16 on its daisy-chain port to determine if the next device in the chain is also a SmartPort device. It has an internal pull-up on the daisy-chain pin 16, and it checks the value on that pin. If it sees 0, that means the next device is also a SmartPort device, and if it sees 1 it means it’s not a SmartPort device or there is no device present. The IIgs may behave the same way, although I’m not sure. So in order for Floppy Emu to be detected as a SmartPort device, it may need to output 0V on pin 16. That’s another potential for two chips fighting. Zap again!

 
Playing It Safe

The question is how to make an Apple II SmartPort mode firmware for Floppy Emu, while avoiding potential electrical damage if you accidentally set the wrong mode on the wrong computer.

Caveat Emptor – I could do nothing. As long as you switched into Apple II mode after connecting the Emu board to an Apple II, and switched into Mac mode before disconnecting from the Apple II, you would never have two chips fighting on the same line. But that’s a poor solution. What if you borrowed a Floppy Emu from a friend for use with your Mac, and you didn’t know what mode it was in? Or you just plain forgot?

Physical Adapter – I could build a small passive adapter that sits between the computer and the Floppy Emu board. An inline resistor of about 100 to 330 ohms on pin 10 should prevent possible damage if two chips were fighting. The same solution would probably also work on pin 16, although correct functioning would depend on the value of the SmartDisk pull-up resistor and the logic threshold of the chip it used to sense the value of pin 16. But I’d really like to avoid requiring an adapter if I can. It would be so much nicer if you could just plug your Floppy Emu straight into your IIgs or IIc and have it work.

Detection – Maybe I could somehow detect whether the attached computer is an Apple II or a Mac, and only switch pins 10 and 16 to outputs after confirming it’s an Apple II. That sounds good, but how? Is there a way with the existing hardware that the CPLD could tell if pin 10 is being actively driven with some value, instead of just floating? Or maybe try to detect a SmartPort reset state before switching the pin directions? I’m not confident the SmartPort reset state won’t also be triggered accidentally by Mac drive logic like the HD20 control routines.

Button Push – I could require the user to push a button to “confirm” Apple II SmartPort mode, each time the Emu board was reset, before switching the pin directions. That would be a little inconvenient, and would still have some potential for damage if you pressed the confirmation button when you shouldn’t have. But it’s the best solution I’ve thought of thus far.