Monday, November 7, 2016

RetroChallenge Epilogue and Apparent Success

After Review of my plan and schematics, I noticed a small error in the Eagle schematic I posted earlier.
  1. Pin 11 of the socket must be connected high (or to pin 12, or to a soft switch to enable video scanning on row E of RAM).
  2. Pin 11 of IC F2 must be disconnected and that line (not the IC pin) must be connected to ground instead (See my hand-drawn schematic).
I figured out problem 1 first. But when I booted I received half of a startup beep repeatedly, like it was stuck. It took a while until I realized I forgot Problem 2 since it was only on my older schematic. I used an IC socket and connected pin 11 and 12 and raised the IC leg. This is important as it decodes whether the Apple II is in RAM or I/O mode. The computer was stuck with no access to I/O or ROM (or simultaneous access to RAM and I/O or ROM).
F2 IC with pin 11 sticking out in a socket with pin 11, 12 connected It boots!

It seems to have worked! The computer shows the APPLE ][ greeting just as expected. I need to connect a keyboard and a disk controller card to test it all out and verify that all 48K is being accessed.

So, I now have an Apple II with 48K of 4164 DRAM in just one bank of RAM. Now for 64K. Or 128K...

2 banks of RAM installed for future 128K modifications

Tuesday, November 1, 2016

Final Push and Failure

I ran jumpers wires on the back of the Apple II+ logic board to route the various signals I needed (PHI_0, AX, A14&A15) to the J1 socket area. Fortunately since only half of J1 is used by the Apple II+, I can use the unused pins of that socket to route my signals through to the new circuit I am adding.

Here is my circuit diagram:

and the adapter board:

Warning - I don't know if this works yet.

After I ran the wires, I started trying to solder a tiny adapter board to translate the 74LS257 signal locations to the new 74LS153 IC. That was taking too much time for something that might not work at all, so I put the circuit on a bread board. Here it is:

Unfortunately, this did not work. I suspect that the dip jumper cable may be too long and adding some capacitance or delay. Or, the RAM timing may have been thrown off by the internal logic of the new IC. My Apple II+ is not booting and is in a similar state to when the RAM was in the wrong row. I'm sure I would have figured this out, but Halloween took priority.

Better news is that my Apple IIb is in a pretty final state. I think I will start buying supplies to actually build it.

Until next time, RetroChallenge!