6502 NOP tester on breadboard

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[digi designer]

In March 2023, I was asked to test a dozen or so 6502/65C02 processors which were likely bad, for go/no-go and otherwise verify they *were* 6502s. I used the opportunity to restore an E&L Digi-tester breadboard product, to set up a breadboarded NOP test circuit. Here's the details of how one of many 8-bit processors can be tested in a simple fashion. - Herb Johnson

E & L Digi Designer breadboard

[digi designer]

On the E&L breadboard, I wired up a 6502 CPU circuit, to test several questionable 65C02 microprocessors. The test is to force a NOP instruction on the data lines. That's sufficient for the processor to run through all of addressable memory to execute NOPs, starting after reset. On reset most 8-bit processors read instructions from a particular address. The test is to monitor high-order address lines to see if they clock (rise/fall) at human-readable rates, with a logic probe LED or lamp.

The 6502 NOP is EA hex, 1110 1010 in binary. A pullup resistor on the data buss represents a 1, ground is a 0. If you look at the resistors from right to left on data lines pins 26-33 (upper right of processor is pin 21), you see three 270 resistors, a green ground wire (E); then resistor-ground-resistor-ground (A). Any resistor of several hundred to a few thousand ohms will do.

The processor clock, is wired to the E&L square-wave generator. The 6502 reset is tied to one of the logic switches. Other 6502 lines are grounded, tied to 5-volts, or left open as needed to run the processor.

There's a few variations among 6502's of a few pins that are unused or grounded in one variation, used in another. Data sheets identify these details. The 6502 isn't a "static" device, it must be clocked at some speed to operate. I found it needed a 10KHz or 100kHz clock to run. CMOS processors like the 1802, can be clocked at low rates to ZERO.

By manually wiring the logic lamp to various 6502 address lines, I can see if the processor is running through the address space by observing the address lines toggle from high to low on a regular cycle.

Results from 13 candidates

[digi designer]

Breadboard tests: two IC's appear to function but not reliably reset. several address lines toggle at progressively slower rates. Possibly useful to test on Apple IIe

one IC very hot!

five IC's don't function as 6502s

two IC's blink briefly on power up, don't reset to run again

two IC's blink but their address lines don't go to logic zero

one has damaged pin 2, mechanically loose on package.

Apple IIe tests: two IC's that functioned on the breadboard, fail reset test on Apple IIe. A few other IC's tested - also fail. Failure means, the IIe on power-up doesn't come up on video to a "Apple IIe" screen. No software, no diskette needed on the Apple II other than its ROMmed codes.

Herb Johnson
New Jersey, USA
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