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I want to purchase a 6502 40 pin CPU in order to verify if my 6502 is faulty, I've seen I can buy from Hong Kong but would prefer closer (to the UK), RS and CPC don’t seen to have any.

Most answers suggest trying a 65c02 processor but these all seem to run at a far greater processing speed than my 6502 clocked at 0.75mHz, Would a 65c02 even work?

My system is the ‘Microtan 65’ system consisting of MT0016, TANEX, TANRAM, ETI Epromer fitted in a 19” Rack brought early 80’s. I’m intending to go though the suggestions and choose which option is best.

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  • Plenty of used 6502's on eBay, eg. ebay.com/itm/1PCS-5PCS-UM6502A-DIP40-IC/… . This one is from a reputable supplier who sells genuine chips, but even the disreputable suppliers generally do too (though refurbishers often don't bother reproducing the exact part number, and sometimes deliberately 'upgrade' it to a faster or more modern variant on the misguided principle that more people will buy fake chips than real ones!)
    – Bruce Abbott
    Jun 6, 2020 at 21:58
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    Good job adding machine specifics to your question. I expanded my answer with a link mentioning the 65C02 in Microtan docs. Also, "Thanks" here is as simple as an up-vote. The format is more like a bulletin board than an e-mail conversation. Everything said in a question or answer is directed at the whole community, and the whole community rates its usefulness via voting.
    – Brian H
    Jun 7, 2020 at 16:12
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    I take on board that I should had been more specific where I had searched for the 6502 and realise not every body would know RS and CPC are UK suppliers, Sorry.
    – Bigmalc40
    Jun 7, 2020 at 17:01
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    Re: lower clock speed, many folks (including me) have been running the 65C02S at 0.0000001MHz (and slower!) with no problems - it’s designed to be stopped indefinitely.
    – Jacob Krall
    Jun 7, 2020 at 23:05
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    Please read up about how this website works. It's not a forum, there are no conversations.
    – pipe
    Jun 7, 2020 at 23:06

3 Answers 3

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You are in luck. The latest version of the 6502 is still manufactured and sold. You need to search for it by the official manufacturer's product code "W65C02S6TPG-14". Western Design Center (that first "W") licenses the manufacture of the chip to various silicon foundries, which wholesale them to dealers you can buy from online. If you don't use this recent product code, you'll find lots of offers for old or NOS 6502 chips whose origins will be more "dicey" than buying a new chip from a local chip reseller.

Incidentally, this latest 65C02 is a fully-static CMOS design rated at 14 MHz maximum, but can be clocked down to <1 MHz. I think it is available in 3 package types, including the "traditional" 40-pin DIP.

NOTE: At the top of page 18 of TANDOC Issue #2, it states: The 65C02 is a direct replacement for the 6502 and it will run all existing programs. This at least suggests use of the old 65C02 in your machine was a known configuration option back in the day. Though, your particular Microtan 65 may need tweaks if certain pins on the modern W65C02S were originally connected (See other answers and comments).

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    Note that if you're looking to replace a part in an existing design that sometimes a CMOS part will not be compatible, the timing of some of the signals will be different.
    – PeterI
    Jun 6, 2020 at 17:55
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    The most important part here is to search for 65C02. This will give hits with several major distributors.
    – Raffzahn
    Jun 6, 2020 at 19:35
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    Not only the timing may be different on more modern 6502s, but also the pinout may have subtle differences! Carefully compare each pin on the data sheet for both the original and your new part to ensure that they're connected appropriately. Typical changes needed to move from an NMOS 6502 or a CMOS Rockwell 65C02 to a Western Designs W65C02S include ensuring that VPB (pin 1) and MLB (pin 5) are not connected (pin 1 is usually tied to GND in older systems) and that BE (pin 36) is tied high.
    – cjs
    Jun 7, 2020 at 6:11
  • I didn't know the TANDOC2 document, so thanks for the information, it is really interesting. Well worth trying the 65c02.
    – Bigmalc40
    Jun 7, 2020 at 16:41
  • The original NMOS 6502 also processes some undocumented opcodes in ways that are common to all general-purpose NMOS variants of that part, but are not available on the CMOS versions. Such opcodes are often exploited in copy-protection schemes, but may also be used to perform such tasks more quickly than would otherwise be possible. For example, when updating a low-resolution tile map on the Nintendo Entertainment System, the unrolled loop "lda zp / sta $2007 / and #$FE / sta $2007" takes 12 cycles per byte pair, but can be cut to ten by exploiting an opcode that stores the "AND" of A and X.
    – supercat
    Jun 8, 2020 at 23:03
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It depends on what you need, really -- and, of course, on where you are.

As others have pointed out, the CMOS version of the 6502, the 65C02, is still readily available. Western Design Center sell them through their distributors, notably Mouser, who ship worldwide. There are, however, minor functional differences between the 6502 and the 65C02. If your application relies on undocumented opcodes, or makes creative use of certain bugs in the 6502, the 65C02 may not work for you.

On the hardware side, three pins have been re-assigned in the WDC 65C02, compared to the 6502 and earlier (Rockwell) 65C02s. You need to ensure that VPB (pin 1) and MLB (pin 5) are not connected (pin 1 is usually tied to GND in older systems) and that BE (pin 36) is tied high.1

Also, the current WDC datasheet for the 65C02 gives V_IH,min (i.e., the minimum input voltage guaranteed to be recognized as "high") as VCC x 0.7, i.e., 3.5V in a 5V system, whereas NMOS or TTL ICs only guarantee a minimum of somewhere between 2.4V and 2.7V on high outputs. This means that the current WDC 65C02 may not work, or may show intermittent faults, in old NMOS / TTL circuits, even though past 65C02s have performed quite well in these circumstances.

These last two issues may be resolved with an adapter board.

That being said, unless you already know it will not work, I would try a 65C02 first.

If indeed you do need a 6502 (without the C) for testing, I see four options for you:

  • Your best bet to acquire a known good chip is probably to buy a working machine that contains a socketed 6502, so that you can then test this 6502 in situ pefore proceeding.

  • You can also look through internet forums (with users from your corner of the world) that deal with 6502s and 6502 machines. Some have market places where people sell tested 6502s; these tend to be more reliable than eBay.

  • If you feel lucky, there are many sellers on eBay and Aliexpress who peddle (ha!) 6502s, but these are often untested, relabeled and/or of dubious origin. They are also cheap -- if you have a known good machine to test them in, or can get your hands on one, you can buy a handful and see if they work. Some probably will. On the other hand, if you had a known good machine, you probably wouldn't be asking here.

  • Finally, there are so-called chip brokers who specialize in sourcing new old stock ICs. A quick check with one here in Germany show that they claim to be able to source several thousand NOS MOS and CSG 6502s. Chip brokers usually don't deal in single quantities, but sometimes, someone organizes a group buy on one of the abovementioned forums.

1 Incorporating a commment from cjs.

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  • 65C02 of different origin (Synertek, Rockwell, (older) WDC) work quite well in TTL environments - at least with Apple II and II+ that is.
    – Raffzahn
    Jun 7, 2020 at 1:11
  • I know. But I still prefer to check the datasheet, and I was surprised to find no value for V_IH,min (i.e., the minimum input voltage recognized as "high") in the current WDC one.
    – Michael Graf
    Jun 7, 2020 at 1:37
  • Yeah, no prop, just adding some experience here. The actual WDC sheets are rather ... well, lets say open to interpretation :) The company has moved toward providing IP, letting such details to be handled by the foundry used.
    – Raffzahn
    Jun 7, 2020 at 1:40
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    Though well hidden, the electrical characteristics are in the datasheet on p. 23 (absolute maximums), p.24 (DC characteristics), p. 25 (AC characteristics), and p. 26 (timing diagram).
    – Alex Hajnal
    Jun 7, 2020 at 4:17
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Mouser UK has them: Western Design Center (WDC) Through Hole Microprocessors. Neither RS nor Farnell (CPC) stock the Western Design Centre microprocessors. The only 8-bit chip you can get from those UK suppliers is the Z80.

Jameco (USA) have the W65C02S6TPG-14, plus a variety of used 6502s for less. Definitely seconding seeing if your application requires NMOS or CMOS. (And if it's for a Commodore, CBM used a bewildering array of 6502 variants, all slightly incompatible.)

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