I'm troubleshooting a dead Apple IIe enhanced, and I'm having trouble finding documentation on testing the 65C02 (as opposed to the 6502).

Specifically, I've found using a logic probe that pin 6 (NMI, active low) is not connected. It seems like that should be high during normal operation.

3 Answers 3


I've been meaning to get around to this for ages, sorry. Tonight I stumbled across SAMS COMPUTERFACTS for the Apple IIe which I had downloaded from the Internet Archive.

Here is the logic chart data for the CPU (IC UC4) from page 27. I've rearranged it for clarity.

Pin 6 should be high, as you say. (Note: I haven't verified this data on real hardware yet.)

21 L  20 L
22 L  19 P
23 L  18 P
24 P  17 P
25 P  16 P
26 P  15 P
27 P  14 P
28 P  13 P
29 P  12 P
30 P  11 P
31 P  10 P
32 P  09 P
33 P  08 H
34 P  07 P
35 *  06 H
36 *  05 *
37 P  04 H
38 H  03 P
39 P  02 H
40 H  01 L

L = Low
H = High
P = Pulse
* = Open (No light On)

NOTE: Logic probe readings taken in Power Up mode, without Disk II Interface Board inserted

  • It looks like the 65C02 in my IIe has lines 20, 22 and 23 pulsing, instead of LOW. I've swapped it out with another 65C02, and the signals are the same, so the problem is somewhere upstream. Those are address lines A11-A13, which all go into a 74LS244. I'll dig into the schematics in the Sam's document and see what the signals should be from there. Thanks for the help! Commented Nov 2, 2016 at 20:13
  • I've dug into the schematics in the Sam's document and checked what the signals should be on the LS244. All are correct, except the three for A11-A13 (8, 15, 17). Would this indicate I need to replace the 74LS244? Commented Nov 2, 2016 at 20:26

65C02 is a CMOS version of 6502 (which was NMOS) chip. Judging from Wikipedia articles on these chips and few of the datasheets I found, it seems like it should work exactly the same way it did on 6502, i.e. held high during normal operation, pulled low to fire an interrupt.

Are you sure it is not connected to anything? I'm not too familiar with Apple devices, but by these schematics it looks like NMI* is held high by 3.3kΩ resistor network and connected to pin 29 of connectors J1-J7: http://www.downloads.reactivemicro.com/Public/Apple%20II%20Items/Hardware/IIe/Schematic/

  • I assume J[1-7] refers to the slots 1-7 for cards in this schematic (at least NMI should indeed be connected to pin 29 on each card slot). More importantly, in the schematic you can clearly see the pull-up resistor that holds it high during normal operation.
    – dirkt
    Commented Sep 14, 2016 at 15:34
  • If the resistor (SIP resistor network in this case, I guess) is not present, then yes, it won't work, so it has to be there somewhere. Was your board damaged mechanically?
    – Algimantas
    Commented Sep 15, 2016 at 18:24
  • 2
    Addition: I have found high resolution photo of the board and had a look at it. Here's the part you should look at. If you have multimeter, measure resistance between contacts I marked and 5V pin on the power connector. You should measure 0Ω between 1 and 2, about 3.3kΩ between 2 and 3 and 0Ω between 3 and 5V pin. If you get these values, I would suspect dead CPU or some device pulling the NMI line low.
    – Algimantas
    Commented Sep 15, 2016 at 20:40

Who made your 65C02? Was it WDC?

If it's WDC, then there are a few pins that should be held high with a 3.3k pull-up resistor. Such as the BE pin which is only on WDC versions. BE (Bus Enable) should be pulled up.

Some example taken from http://wilsonminesco.com/6502primer/MysteryPins.html

VPB (or VP):
With WDC's 65c02, leave it unconnected.  With anyone else's, ground it. 

...pull this up to +5V through a 3.3K resistor.  
You'll only find this pin on WDC's 65c02's anyway, not on those of other manufacturers.

Point is, 65C02 isn't always just a drop-in replacement. You need to make sure all of the pins are grounded, pulled-up (or down), etc. I don't have the A2 schematics in front of me...but make sure that all of the pins are taken care of even if there isn't a card in the system.

Also, this is another good reference on IRQ/NMI of the 65C02. I highly recommend reading the entire "primer".


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