MOS 6522 replacement

Last Updated on February 4, 2022 by Dave Farquhar

One question that comes up pretty frequently among vintage computer enthusiasts is what to use as a MOS 6522 replacement. Commodore used this chip extensively, but so did others. There are suitable replacements in the form of second-source 65c22 chips, but not all of them work. Here’s what to use.

Commodore went out of business in 1994, and started winding down its chip production as early as 1992. But some of its designs were available through second sources, and the 6522 was one of them.

The Western Design Center W65C22N

MOS 6522 replacement
I purchased these vintage 65c22s made in 1985 recently. But you can also get modern replacements that were made this year, if you wish, and the price difference isn’t very much.

If you want a recently-produced chip available through reputable suppliers brand new, the Western Design Center W65C22N in a DIP-40 package is the thing to get. There’s a quirk with WDC’s 6522 variants. The N variant is the one you want for a vintage machine. There’s a variant that ends in the letter S that changes the behavior of pin 21 of the chip. This allows new 6502 designs to run at clock speeds of 14 MHz, but breaks backward compatibility. Get the variant that ends in N, and in the 40-pin DIP package. The link I provided above takes you to what you need.

Aside from a bugfix and being made on a much more modern chipmaking process, the W65C22N is compatible with the old MOS 6522, making it suitable for use in the Commodore VIC-20, Commodore 1541, and other machines that used a MOS 6522 or a variant from Synertek, CMD, GTE, or the like.

Replacing a pair of 6522s with a pair of WDC 65C22s cuts your unit’s power consumption by a little over 100mA. You won’t see any functional improvement, but cutting your 1541 or VIC-20’s current draw by 5 percent is worthwhile.

If the name Western Design Center sounds familiar, it’s where MOS engineer Bill Mensch went after leaving Commodore.

Vintage 6522s and 65C22s for MOS 6522 replacement

Thanks to Irving Gould not understanding what business he was in, MOS Technology didn’t keep up with its peers when it came to modernizing its chipmaking processes. This wasn’t a huge problem in the early 1980s when they were only a few years behind Intel and the others, but the gap widened through the 1980s and eventually became a huge liability.

Commodore never second-sourced the chips in the C-64 like the 6526 CIA and the ultra-famous 6581 SID sound chip. But the 6502 CPU and several supporting chips like the 6522 VIA were. And Western Design Center wasn’t the only licensee. Several companies produced 6522 chips and 65c22 chips in the 1980s. The “c” variants were made on a newer CMOS process that runs cooler and uses less power. Commodore used Rockwell 65c22s in its 1570 and 1571 disk drives to help them run cooler.

Proper 6522s turn up on Ebay fairly frequently, but the selection can be limited. When it comes to those vintage 6522s, there were a number of second sources, and they were all compatible. Also be careful of the price. You can easily end up paying more than the cost of a new WDC chip.

Vintage 65c22s are easier to find, particularly Rockwell chips. Just be careful to get a DIP40 version of the chip, not a PLCC. You’ll also want to be careful of your supplier. You can get a good deal on a quantity of them from overseas, but at some risk of getting remarked chips that may or may not work and may or may not be what they say they are.

I picked up a pair of NCR 65c22s manufactured in mid 1985 recently, because I like the idea of using 1980s parts in my 1541s when possible. Period correctness, you know.

Just watch prices on the old parts. New ones cost $7 each. Shipping on two chips is $8, bringing the price to $11 per chip. You can easily end up paying that much for an old chip with an uncertain history.

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